Abstract: To study the influence of heterogeneous soil-rock mixtures on the ground penetrating radar reflection waves of subgrade voids, an elliptical dilation deformation using a scaling factor function was employed to create shapes resembling natural block stone particles. A random placement and growth algorithm was used to reconstruct a mesoscopic model of soil-rock mixtures with voids. A method to evaluate the heterogeneity of soil-rock mixtures was proposed. Using the Finite-Difference Time-Domain algorithm, the propagation process of electromagnetic waves in soil-rock-filled subgrade voids was simulated. The electromagnetic wave propagation characteristics were quantitatively calculated, and the influence of block stone content, porosity, and moisture content on void reflection signals was revealed. The results showed that the wave field snapshots and profiles of electromagnetic wave signals in soil-rock-filled subgrade exhibited random disturbance characteristics, with the degree of disturbance increasing with greater heterogeneity. Block stone particles and voids formed electromagnetic wave reflection interfaces, enhancing the amplitude of void signals. A certain amount of block stone content could enhance the void signal amplitude, but exceeding a certain content would attenuate the void signal. Porosity and moisture content affected the amplitude and two-way travel time of void reflection signals by influencing the dielectric constant of the subgrade medium. This study provides theoretical and methodological support for the identification and interpretation of voids in heterogeneous soil-rock-filled subgrades.