Abstract: In order to improve the thermal stability of excitation sources and ensure the beam quality of 4kW axis fast flow CO₂ laser, the thermal dissipation mechanism of heat sinks of laser excitation sources theoretically was analyzed by means of computational fluid dynamics method. The heat sink structure of the excitation source with heat flux of 10⁶W/m² and area of 16cm² was optimized. The results show that the maximum temperature on the surface of the heat sink is lower than 340K after optimization. It can fully meet the requirements of the heat source of MOSFET which is the core power of the excitation source in the normal operation of the laser. At the same time, by numerical simulation, the optimal structure size of the microchannel heat sink is obtained. The channel spacing P is 0.6mm, microchannel groove angle θ is 45°, and channel spacing s is 0.1mm. When the Reynolds number is Re=546.9, the heat sink has the best cooling effect. And the stability of laser output power can be controlled within ±2%. This study provides the theoretical guidance for the design of microchannel heat sinks with high heat dissipation capability.