Objective To explore the effect of rehydration guided by the inferior vena cava collapse index (IVC‑CI) on hypotension after subarachnoid anesthesia in elderly patients with hypertension. Methods A total of 120 elderly patients with hypertension undergoing elective hip replacement surgery in Suining County People's Hospital were selected and divided into a control group (group C) and intervention group (group E) by random number table method, with 60 patients in each group. Groups C and E were subdivided according to whether the IVC‑CI value was less than 35% at the time of entry. Group C1 (n=22) and group E1 (n=24) were defined as IVC‑CI<35%, and group C2 (n=38) and group E2 (n=36) were defined as IVC‑CI≥35%. After all patients entered the operating room, and the preoperative fluid physiological requirements were supplemented according to the 4‑2‑1 rule. The ultrasound results of group C did not affect fluid rehydration. In group E2, the IVC‑CI value was reduced to below 35% by accelerating fluid rehydration, and then subarachnoid anesthesia was performed. In groups E1 and C, the infusion rate of 10 ml·kg−1·h−1 remained unchanged, and then subarachnoid anesthesia was performed after 30 min of volume expansion. The four groups' general data (gender ratio, age, body mass index (BMI), cardiac function classification, American Society of Anesthesiologists (ASA) classification, basic IVC‑CI distribution and history of hypertension,operation time, blood loss and urine volume) were compared. The levels of mean arterial pressure (MAP) and heart rate (HR) at entry (T0), before anesthesia (T1), 1 min after anesthesia (T2), 5 min after anesthesia (T3), 10 min after anesthesia (T4), and 30 min after anesthesia (T5) were monitored and recorded. The level of IVC⁃CI at T0 and T1, the incidence of hypotension within 30 min after subarachnoid anesthesia, the number of active cardiovascular drugs used, the infusion volume before and within 30 min after subarachnoid anesthesia， lactic acid (Lac) as well as arterial partial pressure of carbon dioxide (PaCO2) at T0 and the end of operation were compared among the four groups. All patients included in the study were divided into the hypotension group (n=46) and normal group (n=74)，according to the definition of hypotension. Logistic regression analysis was used to explore the related factors affecting the occurrence of hypotension after subarachnoid anesthesia. The diagnostic performance of IVC‑CI in predicting hypotension after subarachnoid anesthesia was analyzed by receiver operating characteristic (ROC) curves. Results There was no statistical difference in the general data (gender ratio, age, BMI, cardiac function classification, ASA classification, basic IVC⁃CI distribution and history of hypertension, operation time, blood loss and urine volume) among the four groups of patients (P>0.05). There was no statistical difference in the level of MAP and HR between groups (P>0.05). Compared with T1, the level of MAP decreased from T2 to T5, the level of HR increased at T3 (P<0.05). Compared with T1, the level of HR had no statistical difference at T2, T4, and T5 (P>0.05). At T0 and T1, there was no statistical difference in the level of IVC‑CI between groups C1 and E1 (P>0.05). Compared within group C1, the level of IVC‑CI in group C2 was higher at T0 and T1, and the level of IVC‑CI in group E2 was higher at T0 (P<0.05). Compared with group C2, the level of IVC‑CI in group E1 was lower at T0 and T1, and the level of IVC‑CI in group E2 was lower at T1 (P<0.05). Compared with group E2, the level of IVC‑CI had statistical differences at T0 and T1 (P<0.05). Within 30 min after anesthesia, the incidence of hypotension in groups C1 and E1 had no statistical difference (P>0.05), and the incidence of hypotension in group E1 was lower than that in group E2 (P<0.05). Compared with group C2, the incidence of hypotension in groups C1, E1, and E2 was lower (P<0.05). More patients in group C2 were using active cardiovascular drugs than in groups C1, E1 and E2 (P<0.05). The infusion volume of group E2 before anesthesia was higher than that of groups C1, C2, and E1. Within 30 min after anesthesia, the infusion volume was lower than that of groups C1, C2, and E1 (P<0.05). There was no significant difference in Lac and PaCO2 between the four groups at T0 and the end of the operation (P>0.05). Logistic regression analysis showed that IVC‑CI (T1) was a primary risk factor for hypotension after anesthesia (odds ratio=2.53, P<0.05), and HR (minimum hypotension after anesthesia) was a protective factor for hypotension after subarachnoid anesthesia (P<0.05). The ROC curves analysis showed that when the IVC‑CI was 35%, the sensitivity of predicting hypotension in patients after subarachnoid anesthesia was 66.79%, the specificity was 84.72%, and the area under the curves was 0.815. Conclusions For the elderly patients with hypertension with IVC‑CI≥35%, the goal⁃directed fluid replenishment with IVC‑CI<35% before anesthesia could reduce the incidence of hypotension after subarachnoid anesthesia. When the IVC‑CI was 35%, it had high diagnostic performance in predicting hypotension after subarachnoid anesthesia in elderly patients with hypertension.