Objective To explore the optimal concentration of ropivacaine in forearm surgery by ultrasound-guided 3-point injection of supraclavicular brachial plexus block. Methods A total of 64 patients with selective forearm surgery were enrolled, and all were anesthetized by the same anesthesiologist using ultrasound-guided supraclavicular brachial plexus block. The local anesthetic was 30 mL of ropivacaine, and 10 mL of ropivacaine was injected into the medial, upper and lateral sides of the supraclavicular brachial plexus by 3-point injection method, with the initial concentration of 0.50%. After anesthesia within 30 min, the blocked effect of 5 nerves (radial nerve, median nerve, ulnar nerve, lateral antebrachial cutaneous nerve, and medial cutaneous nerve) of the forearm was assessed. By sequential increase and decrease method, if all 5 nerves were completely blocked, the next case would reduce the concentration of ropivacaine by 0.05%. If one nerve block was incomplete or ineffective, the next case would increase ropivacaine concentration by 0.05%. The median effective concentration (EC₅₀) of ropivacaine was calculated by logistic analysis statistics. Results Sixty-four patients with forearm surgery underwent ultrasound-guided supraclavicular brachial plexus block. The basic vital indicators were maintained in the normal range before and after nerve block. The block anesthesia was effective in 34 cases and ineffective in 30 cases. The EC₅₀ of ropivacaine was 0.518% (95% confidence interval 0.496%-0.547%). Horner syndrome occurred in 2 patients, and no patients had complications such as pneumothorax, local anesthetic poisoning, vascular injury or dyspnea. Conclusion With 3-point injection method, the ropivacaine dose is 30 mL, and its optimal concentration is 0.518% in ultrasound-guided supraclavicular brachial plexus block.