A user terminal transmits a transmission instruction including an initial time of a first video file to a network node. The network node generates a header of the on-demand video file according to the header of the first video file. The network node fetches a plurality of corresponding video samples from the media data box of the first video file according to the instruction and the media data box of the first video file. The network node fetches corresponding metadata of the video samples from the movie box of the first video file. The network node generates a movie box of the on-demand video file according to the corresponding metadata of the video samples. The network node transmits the header, the video samples and the movie box of the on-demand video file to the user terminal.