Abstract: In view of the piezoelectric technology application in rail detection, which is limited in testing speed and could hardly detect the surface flaw, an online rail flaw detection device based on electromagnetic acoustic surface wave has been investigated. The device uses pitch-catch probes and excites surface waves with two different frequencies, to detect the rail. Surface wave is excited by high power transmitting circuits and received by low noise receiving circuits. Control of the device, acquisition and storage of the signal are accomplished by combination of FPGA and ARM. The software on computer is developed to process data and display results. Experiments showed that the device could detect core defects in the railhead and man-made surface flaws efficiently, which indicated that the device had advantages of independent of couplant, high testing precision and speed.