Abstract: The effects of residual pollutants such as paint, oil, iron chips, floating rust and bubbles on the sensitivity of guided jet ultrasonic testing were studied. The results show that iron chips and bubbles will greatly reduce the detection sensitivity and even cause the echo to disappear. An auxiliary device is designed for the guided jet ultrasonic testing system, which can remove pollutants, avoid bubbles on the surface of the steel plate before testing, and realize the effective on-line automatic testing of the steel plate.