In order to accurately depict the randomness of the node behavior of the P2P file-sharing systems, a dynamic model of the P2P file-sharing systems based on the online-probability is proposed. Firstly, we introduce the online-probability of the nodes and analyze the process of the evolution of the systems. Furthermore, we propose a dynamic model of P2P file-sharing systems based on the online-probability. Through the model, the factors that influence the system are studied. These factors are embodied in the central policies of the system. The relevant algorithms of the dynamic model of the P2P file-sharing systems is improved. The relevant algorithms based on the online-probability are proposed. These policies include peer selection policy, bandwidth allocation policy and peer choking policy. Finally, some experiments are carried out to validate the model.