To solve the abstractness and communication difficulties of dynamic fault tree, a distribution method based on Petri Net for system safety index was proposed by utilizing the advantages of Petri Net including intuition, easy and wide application. Failure rate was selected as a safety index through arranging the safety index and related reliability index. In the comparison of the modeling methods between dynamic fault tree and Petri Net, the latter was chosen to establish index distribution models for static and dynamic logic changes. On the basis of this, the hash classification for the refinement of system safety index distribution method was proposed. And through example analysis, a Petri Net hierarchical model for system faults was constructed to distribute index. Distribution results demonstrate that all the distributed values are within the corresponding safety indexes and this method can overcome the defects of non-intuition in dynamic fault tree method and of excessive roughness in equal distribution method, which provides references for the design and evaluation of aircraft safety.