Abstract: Water cooled substrate products are important components for heat exchange in IGBT modules of rail trains. To calculate their welding rate, taking ultrasonic C-scan images as the research object, the threshold calibration of defect waves was carried out using standard leakage holes, so that the C-scan defect images corresponded to the size of the defect to be identified. Image processing software was used to complete the production of standard graphs. By using the proportional relationship between the number of pixels and the area, the calculation of welding rate was converted into a ratio calculation. By using pixel recognition software, the number of pixels in the well welded area of the ultrasonic C-scan image was statistically analyzed to obtain and calculate the welding rate using the C-scan image.