Karst and goaf have become a hot and challenging problem in expressway construction. To accurately detect the spatial distribution and geometrical form of the karst and goaf zones under the highway, an electrical resistivity constraint inversion imaging method by using prior information obtained by the ground penetrating radar method (GPR) was employed. The conventional ERT inversion method, the GPR, and the ERT constraint inversion imaging method with prior information were used to numerically simulate common karst and mined-out geoelectrical models. Then the imaging laws of various methods in different types of karst and goaf were analyzed and summarized. Numerical simulation results show that, compared with conventional ERT inversion and GPR results, the spatial morphology of anomalous bodies using the ERT constraint inversion imaging method is closer to the real geoelectric model. It can more accurately identify the boundary. Subsequently, a model test study of karst and goaf was carried out in Zhangqiu, Shandong, and the distribution of karst and goaf zones was successfully ascertained, verifying the detection effect of the comprehensive detection method on karst and goaf areas. The proposed method provides a basis for the treatment plan of the karst and goaf under the highway foundation.