In order to get the risk on the level of system and to obtain the quantitative decision reference of safety, this paper proposes an analytic method of system safety based on probabilistic risk. This method will divide the system into different subsystems according to some criterion, count the danger sources of all subsystems and build causality diagrams corresponding to danger sources, and at last compute the probabilistic risk on the level of system according to some rules. The difficult problem of computing the probabilistic risk by this method is discussed.