Based on the theory of acoustic elasticity,the residual stress of butt joint of 12 mm thick Q345 C steel was measured by critical refraction longitudinal wave method. During the test,a 2.5 MHz frequency probe was used to get the residual stress distribution in different regions of butt joint. The ultrasonic propagation velocity will be affected in the process of spreading by the size,morphology and microstructure of the material,based on different regions of welded joints were calibrated by the stress coefficient K value of each area,improve the residual stress of the joint test precision welding of Q345 C steel. By modifying the influence of the microstructure difference on the ultrasonic velocity,the engineering application of the critical refraction longitudinal wave method to test the residual stress is promoted.