The invention aims at providing an automatic detection method of an oblique incidence body wave technical steel plate, which carries out comprehensive and fast detection to steel plates with the thickness of less than 75mm and accurately detects various defects, and a device thereof, and the device consists of an electromagnetic ultrasonic detection unit and a circuit system. The invention adoptsthe oblique incidence body wave mode to carry out detection to the steel plates, which not only is more sensitive to the detect in size, but also can detect the defects of cracks and the like, thus having more comprehensive detection result. The detection unit adopts a mode that transverse arrangement and longitudinal arrangement are combined, thus being more sensitive to the defects of the transverse and the longitudinal direction. The invention combines an ultrasonic transmission method and a reflection method, which not only has higher sensitivity to the defects, but also can position the defects precisely, thus having higher confidence level of the detection result. The invention has wider range for detecting the thickness of the wall of the steel plate, with the highest of 75mm, thusmeeting the requirements for the steel plate with thick wall.