Abstract: This study focused on the enzymatic properties of the protease produced by Brevibacillus borstelensis S8, and constructed an ultrasound assisted enzymatic system for yak meat tenderization. SDS-PAGE was used to verify the purification effect of protease; Single factor experiments were used to explore the enzymatic properties; Response surface experiments were used to optimize the yak meat tenderization process. Subsequently, the tenderization effect of yak meat was characterized by shear force, water holding capacity, myofibrillar fragmentation index, and texture characteristics. The results showed that the purified protease exhibited a single band with a molecular weight of approximately 47 kDa by SDS-PAGE electrophoresis. The investigation of enzymatic properties showed that the enzyme activity was optimal under pH 7.0, 30~40 ℃, and with 20 mmol/L Mg²⁺ supplementation. The optimal process parameters for ultrasound assisted protease tenderization of yak meat obtained through response surface optimization experiments were as follows: protease concentration 2.5%, ultrasonic temperature 34.6 ℃, and treatment duration 39.6 min. Under the above conditions, the shear force of yak meat decreased by 63.3%, steaming loss decreased by 33.7%, centrifugation loss decreased by 52.0%, myofibrillar fragmentation index increased by 47.3%, myofibrillar protein was degraded, and the texture characteristics were significantly improved. This study provides a reference for the development and utilization of protease from B. borstelensis S8, and also provides new protease resources and technical support for yak meat tenderization process.