In order to study the influence of the gas flow channel structure on the output performance of the flowing-gas diode pumped alkali vapor laser (FDPAL), we established the FDPAL theoretical model based on the gas heat transfer, fluid mechanics, and laser dynamics process in FDPAL using side pumping Rb vapor FDPAL (Rb-FDPAL) as the simulation object. The impacts of the gas flow direction, the cross-sectional area and the shape of the runner on the Rb-FDPAL’s output performance were analyzed. The results show that with the horizontal flow method and by increasing the cross-sectional area of the flow channel and setting a masonry structure as the connection between the gas flow channel and the steam pool, we effectively suppress the vortex in the vapor, increase the gas flow rate, and decrease the thermal effect of the steam pool. Rb-FDPAL's laser output power and slope efficiency are higher, and the simulation results are consistent with the experiment.