Abstract: For brazing welding in the presence of inclusion and incomplete fusion defects, water ultrasonic C scanning method was used to evaluate the rate of brazing leaching. Signal processing and image processing knowledge by different thresholds were used to deal with the echo of the solder layer, thus generating different threshold C scan images, and the brazing rate was calculated. The dissection method was employed to verify various C imaging of the brazing rate, and the best threshold coefficient was obtained. The results show that while using the ultrasonic C scan images by Hilbert transform at the threshold for -6 dB imaging, the resulting brazing rate is much closer to the actual value.