Abstract: The presence of stress on key parts of the machine tool will aggravate its friction and wear that seriously affect the machining accuracy and service life of the machine tool, and it is very important to detect the stress on the surface of the structure. In this paper, the detection principle of ultrasonic critical refractive longitudinal wave is explored, and the influence of various factors on the reflection coefficient of acoustic jet lag is analyzed. According to the material parameters of the test piece, the angle of the wedge block was designed; Ten standard experimental pieces were designed and processed, and the acoustic time difference reflection coefficient of ten test pieces was measured by tensile testing machine and ultrasonic testing device. The reasons for the different acoustic jet lag coefficients were analyzed, which provided theoretical and methodological support for the detection of surface stress based on ultrasound.