In peer-to-peer(P2P)video on demand(VoD)system,once the P2P downloading rate cannot satisfy the need of playback,server is quickly referred to for help in providing enough bandwidth.Thus the switch of downloading from other peers(P2P)and server(HTTP)exists.This paper uses the proportion of P2P downloading amount(PPDA)during the video watching process to measure server load.This article is interested in finding a better strategy or switch rule between P2P and HTTP downloading for saving server bandwidth.The authors suggest and model a kind of switch rule based on local buffer amount,using mathematical theory of Brownian motion.It can effectively alleviate the impact of P2P rate fluctuation,reduce the switch times and improve the PPDA by at least 3%-5% on the basis of the former switch rule,which means substantial cost can be saved.Particularly the PPDA is related with the ratio of playback bit rate to the HTTP downloading rate which means the PPDA can be restricted by controlling the ratio in the real-world system.Though the result comes from constant bit rate(CBR)video supposition,it provides perspective and method for variable bit rate(VBR)application,and valuable insights for the future development of P2P VoD system. Once the P2P downloading rate can not satisfy the need of playback, the server is quickly referred to for help in providing enough bandwidth. Here the switch of downloading from other peers ( P2P) and server (HTTP) exists.This paper uses the proportion of P2P downloading amount (PPDA) during the video watching process to measure server load. This article is interested in finding a better strategy or switch rule between P2P and HTTP downloading for saving server bandwidth.The authors suggest and model a kind of switch rule based on local buffer amount, using mathematical theory of Brownian motion. I could substantially reduce the impact of P2P rate fluctuation, reduce the switch times and improve the PPDA by at least 3% -5% on the basis of the former switch rule, which means substantial cost can be saved. Particularly the PPDA is related with the ratio of playback bit rate to the HTTP downloading rate which means that the PPDA can be restricted by controlling the ra tio in the real-world system. How the results comes from constant bit rate (CBR) video supposition, it provides perspective and method for variable bit rate (VBR) application, and valuable insights for the future development of P2P VoD system.