The stability of laser output power and beam pointing stability of the long optical path transmission in high-power laser devices have strict requirements, which puts high requirements on the stability of support mirror mount to maintain the position of the optical components. In order to maintain the beam collimation, the support structure must take into account both adjustability and structure stability, whereas adjustability will introduce instability. This paper proposed a structural improvement design for the commonly used adjustment structure in adjustable support mirror mount. By adding a precise sliding fit which plays a guiding role alone, the function decoupling of the thread matching of the guide and transmission coupling in the traditional adjusting structure is realized, which reduces the influence of large radial clearance on the guiding accuracy in the screw fit, improves the stability of the mirror mount, and builds a stability comparison test device. This device can be used to test the stability of different optical mirror mounts in the same optical path. Experimental results show that the improved adjustment structure could improve the stability of the overall support mount.