Edge-emitting semiconductor lasers have significant differences in the direction of fast and slow axes, and have inherent astigmatism, which seriously affects the transmission distance of underwater communication. Therefore, a set of underwater laser optical antenna system consisting of fast and slow-axis collimators and Galilean beam expander mirrors is designed based on the Gaussian beam characteristics and collimated beam expanding principle in this paper, and the simulation tests of energy loss and divergence angle are also carried out. According to the test results, the divergence angle in the direction of the fast and slow axes of the semiconductor laser is compressed from 49 ° and 9 ° to 0.315 mrad and 0.180 mrad, respectively. And the antenna system can achieve underwater propagation over a distance of 100 m. The difficulty of calibration and fixation of the fast and slow axis collimating mirror in practice is reduced, with a Z-axis tolerance of within 20 μm for the fast and slow axes, and a detector power attenuation of less than 3.9%. This design is compact, easy to process and assemble, with high practical application value, which helps to solve the problems of severe energy loss and low coupling efficiency in underwater long-distance propagation.