A conventional method for modeling radio wave propagation in troposphere is the discrete mixed Fourier transform algorithm (DMFT) based on central difference formula, which will cause the numerical oscillations when calculating the problems over mixed dynamic impedance boundary. This paper describes the forward-backward difference DMFT, which solves the problem of blind spot in traditional algorithm that makes the method more robust and efficient. Compared to traditional DMFT and Miller-Brown model, the correctness and applicability of the new method are verified by simulation results in the complex environment include different sea surface roughness and real terrain.