Abstract: The quality of annular welding line by the high energy beam in the transmission gear directly influences the whole vehicle security. Based on the principle of ultrasonic testing, a ultrasonic C scan testing system for the annular welding line in the gear wheel was designed. The ultrasonic probe, the host and testing software were described. The test result and practical application in factory indicated that the system could detect the equivalent defect of a 0.5 mm flat bottomed hole, and the C scan measurement error of the welding penetration was not more than 0.2 mm. The system provided an efficiency means for quality assurance of the gear wheel.