The invention discloses a system and a method for encrypting and decrypting MPEG (Moving Picture Experts Group) video based on position files. The system comprises a video server, a secret key packet and a client. The invention is characterized in that: the video server is internally provided with a video request processing mechanism, a position file generating mechanism, an encrypting control mechanism, an encrypting mechanism and a data transmission mechanism. The video request processing mechanism receives video requests transmitted by a user, extracts the video file name requested by the user and transmits the video file name to the encrypting control mechanism; the position file generating mechanism generates position files corresponding to the video file according to the video file and transmits the position files to the encrypting control mechanism; the encrypting control mechanism receives the video file name provided by the video request processing mechanism, receives the position files provided by the position file generating mechanism and obtains corresponding video files according to the video file name and the position files; and the encrypting mechanism encrypts the corresponding video files. The invention has high efficiency, safety and flexibility.