A system for providing Video-on-Demand (VOD) service in a network includes a plurality of user devices and a server. Activation of an allowed transmission mode or a peer-to-peer transmission mode is dependent on whether a peer-to-peer transmission function of a first user device of the user devices is limited or not. The first user device calculates respective transmission rates of the two transmission modes to determine their respective download ranges. The transmission mode having a higher transmission speed downloads data close to a start of the target data and another transmission mode having a lower transmission speed downloads data close to an end of the target data. Based on the target data temporarily stored in other user devices, the first user device assigns each of the two transmission modes a download sequence of a plurality of clips of the target data to download the clips of the target data.