To investigate the factors that affect incentive mechanism of BitTorrent system,this paper makes a modeling analysis.Because there are random selections,we use Markov process to model the incentive mechanism.We first get the transition probability matrix and then calculate the limit of the transition probability matrix.Theoretical analysis and experiment results show that peers with same upload bandwidth cluster,and adjusting randomly selected nodes can affect the speed of clustering.The model is consistent with the incentive mechanism,and by adjusting parameters we can improve the performance of the system.