The practical formulas for Gaussian Beams method are theoretically derived. The properties of Gaussian beams are investigated. The transmissive laws in free space and the reflective laws of Gaussian Beams are mentioned. Also the choice of initial value of the Gaussian Beam and the offset phase for reflection in Gaussian Beams Method are investigated in this paper. These will be very useful in the application of Gaussian Beams Method in three-dimensional structures.