Abstract: In satellite laser ranging, in order to meet the ranging requirements of different range targets, it is necessary to adjust the divergence angle of laser beam. Usually, the defocused beam expander system is used to change the distance between two lenses and adjust the divergence angle, which will produce defocused aberration and affect the laser beam quality and the ranging ability of the system. Therefore, it is important to study the influence of defocusing aberration on laser beam quality and system ranging ability. Firstly, the transformation effect of the beam expander system on the fundamental mode Gaussian beam is analyzed, and the relationship between the angular magnification and the defocusing amount is obtained. Secondly, based on the propagation theory of fundamental mode Gaussian beam, the influence of defocusing aberration on the spatial distribution of beam energy density is obtained. Finally, for satellites with different distances, the number of photons received by the laser ranging system with and without defocusing aberrations is calculated. The simulation results show that the angular magnification and defocusing amount obey a linear relationship approximately, the defocusing amount changes 1mm and the divergence angle changes 1μmrad; The defocusing aberration affects the energy density distribution of the beam and makes the spot diffuse; For satellites with different distances, the number of photons received by the ranging system without defocusing aberrations at different angular magnification is about twice that of the system with defocusing aberrations, which can effectively improve the ranging capability of the system.