Abstract: In order to achieve laser output with high quality beam, one new method to simplify the calculation of diffraction loss of different order transverse modes was used by theoretical analysis and numerical calculation. The results show that, when Fresnel number of different transverse modes N>0.7, the relative error between simplified calculation and accurate calculation is less than 0.2%. The diffraction loss of different order transverse modes calculated by the new algorithm can meet the sufficient accuracy. The new algorithm is reasonable and feasible. The sorting of diffraction loss of laser transverse modes carried out by the simplified algorithm rapidly can provide theoretical guidance for the design of high beam quality lasers.