Abstract: In order to realize the real-time monitoring of train's running state and ensure the safety of train operation, a method to monitor the wheel-rail contact condition using ultrasonic technology was proposed. The W-M fractal function was used for mathematic description of wheel and rail surfaces, and a finite element model of wheel-rail contact monitoring were built with the multiphysics software. Using the model, the proposed monitoring method, the roughness of interface, and the types and thickness of the media layer were simulated, and the sound pressures of the wave reflected from the contact interface were obtained under different conditions. The simulation results show that the proposed method of monitoring the wheel-rail contact condition by detecting the ultrasonic sound pressure level is effective and feasible. When the ultrasonic frequency equals 14 MHz, the rail surface roughness can be detected with the best distinction degree. When the frequency is larger than 14 MHz, the third medium thickness has a good distinction degree. When the frequency is constant and the thickness of the third medium is large enough, the sound pressure level of reflected ultrasonic wave tends to a constant value 102 dB.