Abstract: Monte Carlo simulation is used in this paper to analyze the initiating event of loss of equipment cooling water system,the frequency of 2.7×10^-3/（reactor·year）^-1 is obtained,and the main contributor is the common cause demand failure of equipment cooling water system pumps and service water system pumps.The traditional fault tree result is 3.8×10^-1/（reactor·year）^-1（the main contributor is independent operation failure of equipment cooling water system pumps/service water system pumps）.This result is more closer to the statistic value 9.7×10^-4//（reactor·year）^-1 in the Nuclear Regulatory Commission（NRC） report NUREG/CR-5750.The reason is that the mission time of the system in initiating event analysis is 1 reactor.year,so the redundant devices and the repair of the fault devices are important factors for the result. And for this type of problem related to the time order,the assumption is too conservative because of lack of time factor in the fault tree,while MC simulation is a dynamic method,and the system behavior can be described adequately,so the result is more closer to the statistic value of the nuclear power plant.