The present invention discloses a method for media file compression, which includes: extracting the encoding parameters from an input media file, separating and decoding the audio and video stream from the input media file, and extracting an original audio stream and an original video stream; computing the transcoding parameters required for compression according to the encoding parameters; encoding the original audio stream to output a new compressed audio stream, and encoding the original video stream to output a new compressed video stream according to the transcoding parameters; synthesizing the new compressed audio stream and the new compressed video stream to create a new media file. The present invention also provides a system for media file compression. By means of the method and system of the present invention, users can realize simple and rapid compression for media files of various formats without understanding media expertise, and the storage space can be saved, so that it is convenient to carry, transfer and share the media files.