The aim of the invention is to provide a flaw detection technology for carrying out comprehensive, rapid and automatic detection on a median plate or a thin plate by electromagnetic ultrasonic horizontal polarization shearing wave, a steel sheet automatic detection method for rapidly and effectively detecting various disadvantages through an electromagnetic ultrasonic SH wave technology and a device thereof. The device consists of an electromagnetic ultrasonic detection unit and a circuit system. The method adopts SH wave to detect the median plate and the thin plate, is sensitive to the disadvantages of the surface and the inside of the steel plate, receives a pure signal and can effectively detect various disadvantages of the steel plate. As the SH wave has small attenuation during transmission, the SH wave has long detection distance, can realize the comprehensive detection of the steel plate with fewer probes and greatly improve the detection efficiency. The method combines an ultrasonic transmission method and a reflection method, can detect various disadvantages of the steel plate, can judge the shapes and positions of the disadvantages, and has high confidence level of a detection result.