Abstract: In order to improve the laser transmitting power of laser detection system, assuming exit laser is a fundamental mode Gaussian beam, the effect of the obscuration ratio of Cassegrain telescope and the waist radius of the incident light beam on the light power transmission was analyzed based on the transformation and transmission characteristics of Gaussian beam passing through the optical system. The numerical calculation indicates that the optical power transmission of the system decreases with the increase of the obscuration ratio and the waist radius of the incident light beam. The best match between the obscuration ratio and the spot size of the transmitting aperture was given. The effect of the defocusing error (installation error) of 0.2mm on the spot size of emergent beam was discussed.