A mathematical modeling method of an off-axis double-reflection laser beam expander is described. At the condition of input a few special parameters, all the parameters on the main operation panel of the beam expander are calculated. With the calculation software such as MATLAB and EXCEL, the initial structure of all the operation panels of the beam expander is obtained easily. We can judge directly with the method whether the initial structure accords with the requirements of system assembling and debugging or not. It has the characteristics of convenience method and fast speed calculation.