For the finite-difference time-domain (FDTD) solution of the half-space problem, the conventional total-field/scattered-field (TF/SF) technique cannot be directly applied to introduce excitation fields for electromagnetic scattering from the ground. In this paper, we extend the generalized total-field/scattered-field (G-TF/SF) method to accurately calculate the high altitude electromagnetic pulse (HEMP) environment in 3D ground scenarios and adopt the convolution perfectly matched layer (CPML) absorption boundary condition around it. The G-TF/SF method effectively eliminates edge effects in numerical calculations for finite ground, thereby improving accuracy in the calculation of the ground electromagnetic environment. Moreover, only the incident field needs to be introduced at the G-TF/SF boundary, avoiding the computation of reflected and transmitted fields.