Abstract: In response to the difficulties in implementing online automatic detection for nuclear power steel plates, the low efficiency of traditional manual ultrasonic testing, and the high labor intensity, this paper designs an automatic ultrasonic detection device for steel plates that can simulate the manual ultrasonic detection process. The device is composed of an omnidirectional travelling flaw detection trolley, a laser positioning guidance mechanism, a multi-functional probe holder, a multi-channel ultrasonic detector, a power supply and coupling water supply unit. It uses servo motors to drive McNamara wheels to achieve omnidirectional movement. Combined with laser guidance and multi-channel detection systems, it completes the full-automatic scanning of the plate surface along the preset trajectory. It is suitable for rapid and automatic detection of steel plates with a thickness of 20-100 mm. Field tests show that the device can effectively detect internal layered and non-layered defects, with detection rate and determination accuracy comparable to manual detection; the detection efficiency is three times of traditional manual detection, and the false alarm rate of defects can be controlled within 3%, which can meet the personalized and high-standard non-destructive testing requirements for high grade steel plates.