Abstract: In view of the void phenomenon under the slab caused by the repeated use of high-speed railway ballastless track slab all the year round, through establishing the finite element model of slab ballastless track and simulating the propagation law of ultrasonic wave in the multi-layer medium of the track, the characteristics of ultrasonic simulation signals received under different void levels are analyzed, the identification parameters of interlayer void between track slabs are constructed, and the numerical simulation results are verified by physical tests. The results show that the interlayer void of the track slab strengthens the reflected echoes of the ultrasonic wave, the signal volatility increases and its decay speed slows down. The signal spectrum shows new peaks at low frequencies and high frequencies. The low-frequency amplitude in the range of 0-15 kHz is related to the local vibration at the void, while the high-frequency amplitude in the range of 30-45 kHz is related to the overall vibration of the structure. By calculating the energy ratio δ in the frequency domain, the void state inside the track slab can be effectively evaluated. With the increase of void degree, δ gradually increases.