Abstract: An on-line EMAT detecting system for wheelset tread based on FPGA was developed to improve the detecting efficiency of wheelset. Combining EMAT with SOPC technology based on FPGA, this system fully took advantage of FPGA in design flexibility and processing speed. It could achieve transmission and reception of the electromagnetic ultrasonic surface wave, data acquisition, weak signal detection, storage and display. Experiments showed that the system possessed high detecting accuracy and efficiency, which could detect an artificial defect that is deeper than 2 mm on wheelset tread when the trains speed was slower than 20 km/h.