In order to realize the precise alignment of normal orientation of mirrors in the long-distance optical transmission path of high-power laser device, installation error sources based on mirror normal orientation are analyzed. As for the structural and installation features of high-power laser device, a precise alignment scheme which combines pre-collimation, module-pose off-line coping, and flexible joining is selected. The off-line collimation and reset precision are verified, and the experimental data confirms the feasibility of the above scheme. This technique has achieved favorable effectiveness in the assembly and debugging of transport system in Shenguang-Ⅲ laser driver.