Since the advent of laser in communication, remote sensing and space observation and so on many fields obtained the rapid development,and the laser emission system is the core composition of various laser applications. This paper briefly introduces the principle of the laser beam expander system. Laser beam expander system through the study of the expanded beam of laser light source transformation, compress the divergent angle, the light source can reduce the energy loss in long distance transmission. This paper compares the optical structure of various common laser beam expanding systems, analyzes the advantages and disadvantages of refraction and reflection, coaxial and off-axis, designs an off-axis three-reaction system, and briefly describes the selection of lens in high-power laser beam expanding system. © 2022 SPIE