Abstract: In order to study the response characteristics of geological radar waves to soil-rock mixed subgrade void, a microscopic model of soil-rock mixture with pores is built with considering the randomness and non-uniformity of soil-rock mixtures. The random placement method together with the random growth method are employed for the model, in which the gravel particle model is built by the elliptical expansion deformation method. The geological radar forward simulations of soil-rock mixed subgrade void are carried out by using GprMax software. And the applicability of the model is verified by the equivalent dielectric constant. Furthermore, the influence of subgrade roadbed gravel content, porosity, and moisture content on the reflection signal of voids are simulated. The results show that the electromagnetic waves will be strongly reflected when it propagates to the void. The electromagnetic wave scattering in the soil-rock mixed subgrade is obvious, the waveform is blurry, and the wave surface is not smooth. The larger the particle size of the gravel particle, the more severe the distortion of the reflected wave, and the lower the distinguishability. Gravel is the main cause of electromagnetic wave scattering, and an appropriate amount of gravel can enhance the amplitude of cavity signals. If it exceeds a certain amount, the signal will attenuate. The porosity and moisture content mainly affect the reflection signal characteristics of void through the dielectric constant. This study can provide a theoretical basis for the recognition of geological radar images of void in soil-rock mixed subgrade.