Millimeter and submillimeter wave has been widely used in the military and civilian. Reflective surfaces are usually analyzed by Geometrical Optics(GO) and Physical Optics(PO) methods. This paper firstly compares these two methods simply, and then specially introduces Diffraction Gaussian Beam Analysis(DGBA) method based on GO. On that basis, this analysis method is improved to analyze the three-dimensional system. The efficiency and accuracy between this method and commercial software based on PO are compared. It could be seen that the three-dimensional DGBA method is an efficient and accurate method to analyze and design three-dimensional quasi-optical network.