Objective To determine the effects of ropivacaine at different concentrations in the same volume on the analgesic effect and diaphragmatic motor function in elderly patients with intermuscular sulcus brachial plexus block. Methods A total of 46 patients who were scheduled for shoulder arthroscopy were selected. According to the random number table method, they were divided a 0.5% ropivacaine 20 ml group (group A, n=24) and a 0.3% ropivacaine 20 ml group (group B, n=22) and received ultrasound‑guided intermuscular sulcus brachial plexus block before induction of general anesthesia. Their hemodynamics, Visual Analogue Scale (VAS) scores, diaphragmatic movement and other adverse reactions were recorded at different time points. Results There was no statistical difference between the two groups in hemodynamics within 30 min after block (P>0.05), as well as in VAS scores at resting and during movement and other adverse reaction rate within 20 h after block (P>0.05). Patients in group A presented significant differences in bilateral diaphragmatic movement on quiet breathing and maximal expiration 5 min, 3 h, 6 h and 20 h after block, compared with those before block (P<0.05). Patients in group B also demonstrated significant differences in bilateral diaphragmatic movement on quiet breathing and maximal expiration 5 min, 3 h and 6 h after block, compared with those before block (P<0.05). The incidence of diaphragmatic palsy in group A was higher than that in group B 3 h and 20 h after block (P<0.05). Patients in group A presented a decreased diaphragmatic thickness fraction at the blocked side 5 min, 3 h, 6 h and 20 h after block (P<0.05), as well as an increased diaphragmatic thickness fraction at the opposite side 3 h, 6 h and 20 h after block, compared with those before block (P<0.05). Patients in group B presented a decreased diaphragmatic thickness fraction at the blocked side 5 min, 3 h and 6 h after block (P<0.05), as well as an increased diaphragmatic thickness fraction at the opposite side 3 h and 6 h after block, compared with those before block (P<0.05). Conclusions Similar analgesic effects are found between 20 ml of 0.5% and 0.3% ropivacaine in elderly patients within 20 h after shoulder arthroscopy, but the incidence and duration of diaphragmatic paralysis in patients of the high concentration group significantly increase.