A new method of EM-scattering computation of site environment based on Fresnel diffraction theory is presented as the essential features of MLS scattering. By the equivalent principle of the Fresnel diffraction through rectangular aperture to the EM scattering of site environment, the indirection relative to direction signals level ratio can be acquired. A unique property of this method is just coherently computing reflection and diffraction. The calculation procedures are simplified. Then, the computed load is further lower. To verify the feasibility and validity, the proposed method is applied to analyze the scattering characteristic in the specific site environment on the airport. Moreover, the results are compared with the calculation results using the common method based on the Geometrical Optical (GO) theory. Comparison of them shows the new method can overcome the limitation and deficiency of the common method in the scattering computation process. What’s more, it has some practical values on determination of the extent of disturbance caused by site environment.