Abstract: Affected by the structure,the electron beam weld with complex structure didn’t have the condition of radiographic testing. However,it could be effectively detected by ultrasonic C-scan testing. The lack of penetration could be located,and the weld penetration could be measured. It was an effective method for the internal quality inspection of weld. Through the simulation of ultrasonic sound field,the probe parameters were optimized to obtain a strong sound field in the weld area. The reliability of the method was verified by ultrasonic C-scan testing of welding specimens,and the testing sensitivity could reach the defect equivalent of φ3 mm-9 d B. The results showed that the ultrasonic Cscan technology was possessed of high detection rate and high sensitivity for the lack of penetration. Combined with the ultrasonic C-scan image,the weld penetration could be detected accurately. Compared with the destructive test,the defect location are accurate,the detection error of weld penetration was about 4%,and the result was reliable.