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Alternating direction implicit difference scheme (ADI-FDTD) divides a traditional time step into two time steps, with forward difference and backward difference, and integrates both the advantages of unconditional stability of implicit difference scheme and relatively simple calculation of explicit difference scheme, it breaks through the constraint of Courand–Friedrichs–Levy (CFL), and is characterized by unconditional stability. And the boundary condition of uniaxial anisotropic perfectly matched layer (UPML) is anisotropic medium PML applied in absorption edge of FDTD area, with the absorption of wide band, simple iterative formula, and easy programming, without the electric and magnetic field splitting. By discretizing the Maxwell equations in a two dimensional structure, numerical formulas of ADI-FDTD with UPML boundary condition for GPR wave are presented in detail. GPR simulations are carried out for two models. Based on the analysis of numerical results, insights of the radar wave spreading and changing in space are obtained, which can provide a better interpretation of real radar data. The results show that the ADI-FDTD algorithm based on the UPML boundary condition can deploy larger time step and eliminate the strong reflection on the truncated boundaries, lead to an efficient GPR modeling.