As the node of peer-to-peer（P2P） storage system is unreliable,it is difficult to predict the overall system availability.In order to solve this problem,a method,which models the whole process of system replica management using Markov chain,is proposed.Then,by analyzing the mathematical model,the system failure probability,the time period,the system life cycle and other relevant data are theoretically calculated.Finally,some simulations are performed on the PlanetLab distributed system platform.The results prove that the theoretical calculation is correct.