Objective Determining the optimal bolus volume for programmed intermittent epidural bolus (PIEB) using ropivacaine 0.08% and sufentanil 0.3 mg/L to provide adequate analgesia in 90% of women during the first stage of labor. Methods A prospective, double‑blind dose‑finding study was conducted on 40 nulliparous parturients with epidural labor analgesia according to the biased coin up‑and‑down sequential allocation method. The PIEB interval was fixed at 45 min with 0.08% ropivacaine and 0.3 mg/L sufentanil. Participants were divided into groups 8 ml, 9 ml, 10 ml, 11 ml, and 12 ml according to the PIEB volume (8, 9, 10, 11 ml and 12 ml, respectively). The PIEB volume of the first parturient was set at 8 ml, and that of the subsequent parturient varied according to the labor analgesia effect of the previous parturient. The primary outcome was the adequate analgesia provided by the bound PIEB volume for the corresponding parturient. Secondary outcomes included the maximum sensory block level, motor block degree in the leg, and occurrence of hypotension and pruritus. The truncated Dixon‑Mood method and the Isotonic regression analysis method were used to estimate 90% practical volume (EV90) and 95% confidence intervals (CI). Results The set PIEB volume provided effective analgesia for 32 parturients. According to the effective analgesia results of each parturient, the EV90 was 10.53 ml (95%CI 9.18, 11.87 ml) and 10.76 ml (95%CI 10.30, 11.82 ml), using the truncated Dixon‑Mood method and Isotonic regression analysis, respectively. A total of 3 parturients presented with sensory block levels above T6 in groups 11 ml (n=1) and 12 ml (n=2). However, none of the parturients presented with sensory block levels above T6 in groups 10 ml, 9 ml, and 8 ml, respectively. There were no cases of hypotension and motor block. There is one parturient complained of pruritus in groups 10 ml and 12 ml, respectively. Conclusion With a fixed 45 min PIEB interval, the EV90 with ropivacaine 0.08% and sufentanil 0.3 mg/L should not be less than 10 ml.