Thermal effect becomes more prominent in the laser gain medium, to overcome this problem, the forced convective heat transfer with reliability and durability is widely used. The hydro-structures of dimensions of the flow channel affect the thermal performance immediately and efficiently. In this paper, with proposed cooling configuration based on longitudinal forced convective heat transfer, the factors of flow rate, state of flow field and surface roughness are investigated. The results reveal that fully developed flow state, higher flow rate and rougher surface lead to a better cooling capability. In the simulation results with 30 L/min flow rate, the calculated averaged convective heat transfer coefficient is as high as 104 W/m2 ·K, and with slightly fluctuations in fully developed flow period.