Abstract: Using tensile testing machine and ultrasonic stress detector, axial tensile was carried out on 45 steel of volute bolt of hydropower unit, and the stress value and the corresponding ultrasonic p-wave and s-wave propagation time were measured in the tensile process, so as to obtain the ultrasonic p-wave and s-wave acoustic ratio, i.e., the ultrasonic p-wave and s-wave velocity ratio. The quantitative relationship between elastic stress and ultrasonic p-wave and s-wave acoustic time ratio was established, which provided a basis for real-time determination of bolt elastic stress by ultrasonic wave. The results showed that the axial elastic stress of 45 steel was within the elastic deformation range of the steel, and the relationship between the acoustic time ratio of ultrasonic p-wave and s-wave and the elastic stress was linear, σ=-50 249.8 S_l/S_t+90 536.1. The error between the stress measured by p-wave and s-wave and the actual stress was 2.36%, which meets the engineering requirements.