Abstract: The source can be spread into a set of Gaussian beams (GBs), whose correlative parameters depend on both shape of reflector and the type of source, and the spread coefficients can be computed by the method of least square or point-matching. The correlative parameters of reflected GBs can be computed by some formulas and the spread coefficients can be gained by solving the current continuity equations. GB method has higher computation speed and the same computation accuracy compared with PO method. In addition, the rate computation of GB method has nearly no relationship with work frequency.