Accurate modeling and efficient evaluation are discussed in this paper for electromagnetic scattering from perfectly conducting object penetrating half-space interface by methods based on mixed potential integral equation.Proper Green's functions for half-space are adopted to satisfy the special boundary condition du to the object penetrating the interface and to simplify numerical discretization.Optimization upon the procedure of impedance matrix filling is made in conjunction with tabulation and interpolation for evaluating the Green's functions to speed the generation of impedance matrix.The proposed methods can be used to accurately solve the half-space problems and on the whole overcome the difficulties resulted from object penetrating the interface.