Abstract: Based on the acoustoelastic theory, the relation between the propagation velocity of ultrasound and the bolt's axial stress was introduced, and the method of combining the shear wave and longitudinal wave was adapted to measure the bolt's axial stress. Based on the introduction of acoustoelastic theory, the bolt's axial stress was detected, and grade 8.8 carbon steel bolt with the size of M30×150 was selected for the test. The time-of-flight of the shear wave and longitudinal wave was obtained by combining cross-correlation signal processing method. The variation trend of elastic coefficient of bolt stress under different stress states was obtained, and the analysis method of piecewise calculation of the bolt's axial stress was conducted. The average error between the calculated results and the theoretical stress value was only 2.026%, which improved the measurement accuracy of the bolt's axial stress.