Abstract: Reflector system is an important part of the laser facility, whose rotation stability has a direct effect on the beam's propagation. Aiming at the stability problem of large-scale precision mirror mount caused by ground random vibration, the formula about the root mean square (RMS) of mirror's rotation angle is deduced. The finite element (FE) model of a reflector system is built in ANSYS, the modal analysis and the micro-vibration calculations are done on the FE model. The RMS rotation angle of the mirror is calculated according to the simulation result. At the same time, another engineering approach to calculate the RMS rotation angle of the mirror is presented by FE simulation. The results of the two approaches are consistent, which shows that the approaches to calculate rotation angle of mirror are effective and reasonable.