Abstract: In Antarctic optical telescope operating in Dome A, Antarctic, some mirrors are exposed to harsh environment when the telescope is observing. Frost snow and ice on the surface of the mirrors degrade the transmission and reflection of the optical system. Telescope mirror defrosting system is aimed to settle this problem. In this paper, a fault tree of the mirror defrosting system based on FTA (Fault Tree Analysis) is established to make sure the system works reliably. After qualitative and quantitative analysis of the fault tree, a minimal cut set of the system is obtained and the structure importance ranking of the basic events causing the fault appears. The analysis results show clearly the bottle neck of the system and point out the way to improve the reliability of the system theoretically.