Abstract: GPR impedance inversion is an effective method to obtain accurately the intrinsic parameters of the subsurface medium. This method relies on low frequency information provided by logging data, but it is rarely accompanied by drilling in practical applications. To solve the problem, the common midpoint velocity analysis is employed to provide more low frequency component information for impedance inversion of GPR to finely reconstruct the dielectric parameter of subsurface in the framework of velocity-constrained inversion technique. Firstly, a layer model is specifically set up as an example to verify the feasibility of the developed velocity- constrained inversion to regulate the initial model. Then, the impedance inversion test is performed on two random media models, result of mean relative error from the true model is 8.73%, which show that the overall structure is more consistent with the real model, and more importantly, the microstructure is also finely depicted. The proposed method is more efficient and economical in impedance inversion of ground penetrating radar with random medium model, with more detailed information contained in imaging results, the method is very feasible and applicable in the estimation of other physical parameters in soil investigation.