In order to understand the effect of gas velocity distribution uniformity on injected energy and output energy of lasers, the flow field in discharge chamber is simulated by computational fluid dynamics (CFD) so that the gas velocity distribution in longitudinal and vertical directions can be acquired. According to the elements that affect the gas velocity uniformity, the whole flow field is optimized. Based on the fluid dynamics, laser structure and light weight requirement, three clapboards are added to improve the flow field distribution characteristics. After optimization, the average gas velocity reaches 99.3 m/s. The velocity non-uniformity in longitudinal direction is 5.2%, and that in vertical direction is 7.1%. The repetition rate of laser increases from 300 Hz to 365 Hz, the injected energy of single pulse increases from 160 J to 171.5 J, the output energy increases from 20 J to 21.8 J, and the divergence angle of laser beams decreases by 0.5 mrad. Improving flow field distribution characteristics in the discharge chamber will enhance the integral performance of lasers.