Objective To determine the effective volume of 0.5% ropivacaine in patients receiving thoracoscopic analgesia with erector spinae plane block (ESPB) by Dixon's up-and-down sequential method. Methods A total of 31 patients who underwent thoracoscopic lobectomy in Yueyang Hospital of Integrated Traditional Chinese and Western MediCIne, Shanghai University of Traditional Chinese MediCIne from Jan. to Sep. 2020 were selected. Ultrasound-guided 0.5% ropivacaine ESPB of corresponding spinal ganglion segment was performed according to the operation site. The initial volume of 0.5% ropivacaine was 35 mL. If the first patient reached the best effective analgesic standard, the drug volume of the second patient decreased by 2 mL; on the contrary, the drug volume increased by 2 mL. The trial was terminated when 7 inflection points of invalid volume appeared. The following 3 criteria were met, and the best effective analgesic standard was achieved:(1) there was no significant fluctuation in vital signs at the beginning of skin inCIsion (the rise of mean arterial pressure and heart rate did not exceed 20% before skin inCIsion); (2) there was no pain-related agitation after extubation; and (3) when no remedial analgesics were used 2 h after operation, the visual analogue scale score of pain was ≤ 3. Probit probability regression analysis was used to calculate 50% effective volume (EV₅₀), 95% effective volume (EV₉₅) and their 95% confidence interval (CI). Results A total of 30 patients were enrolled, including 21 males and 9 females, aged (60.0±6.8) years, with body mass index of (22.7±3.1) kg/m² and American SoCIety of Anesthesiologists gradeⅡ(9 cases) orⅢ(21 cases). Eighteen (60.0%) patients achieved the best effective analgesia. The EV₅₀ of 0.5% ropivacaine was 23.6 mL (95% CI 21.8-25.0 mL), and the EV₉₅ was 26.9 mL (95% CI 25.3-35.2 mL). No complications such as bleeding, hematoma, local anesthetic poisoning, infection or nerve injury occurred in the patients. Conclusion EV₅₀ and EV₉₅ of 0.5% ropivacaine in patients receiving thoracoscopic analgesia with ultrasound-guided ESPB are 23.6 and 26.9 mL, respectively.