Abstract: The multi-state reliability of reactor purification system was analyzed based on the universal generating function (UGF). The reliability of the purification system and the probability of the system at different performance levels were obtained. The results could well reflect the relationship between the system’s performance and reliability. Meanwhile, the effect of common cause degradation and common cause failure (CCF) were taken into account. The results indicate that this approach is more practical compared to the traditional approach only considering the dual-state component.