Backgroundː This prospective, single-armed study investigated the median effective volume (MEV) of 0.375% ropivacaine for ultrasound-guided anterior suprascapular nerve block (aSSNB) required to produce an analgesic effect on patients undergoing arthroscopic shoulder surgery. Our primary objective is the MEV. The secondary objectives are the 24h sufentanil consumption, 24h PCA Presses, and diaphragm movement.Methodsː 24 patients were enrolled, and 23 completed the study. Using an up-and-down procedure, patients enrolled in the study received different doses of 0.375% ropivacaine in the anterior section of the suprascapular nerve. The aSSNB was performed under ultrasound guidance. The first patient received 15 ml of 0.375% ropivacaine. After a successful or failed block, the next patient would receive a volume decreased or increased by 3 ml. 23 patients were recruited in the study and then met the stopping rule. We used a 3-point scale to determine if the block is successful or not. We considered a successful block if both the sensory score of C5 and C6 was equal to or greater than 1. Otherwise, the block was considered a failure.Results: MEV50 was 6 ml (95% CI 5.78-6.78 ml), and MEV95 was 13.88 ml (95% CI 13.37-14.87 ml). There was no significant difference in the patient-controlled analgesia (PCA) presses, 24h sufentanil consumption, and diaphragm movement between successful and unsuccessful blocks.Conclusionsː To produce an analgesic effect, the MEV50 is 6 ml, and the MEV95 is 13.88ml in patients conducting aSSNB using 0.375% ropivacaine for analgesia undergoing arthroscopic shoulder surgery.Trial Registration: ChiCTR2200061773. 02/07/2022 retrospectively registered.