Abstract: In the laser engineering system, a beam expander and an annular aperture were taken as the equivalent of the Cassegrain telescope system. The far-field irradiance distribution of an expanded Gaussian beam which had gone through an annular aperture was deduced in an inner optical system. The energy loss and the effects of the ratio of Gaussian beam-waist radius after expander to outer radius of the annular aperture on Strehl ratio were numerically computed. The results show that an optimal ratio exists for least energy loss and high Strehl ratio on the condition of the definite parameters of the Cassegrain telescope.