The invention relates to a multiplex device and a multiplex method in a communication system. A video and audio multiplex device of the invention receives video and audio data and then builds the video and audio data into multiplex frames for multiplexing, and comprises a video and audio preprocessor, a cache manager, a video frame buffer and a multiplex framer, wherein, video frame data in the video and audio data sent by the video and audio preprocessor is stored into the video frame buffer by the cache manager and the video frame with a certain data size is taken out from the video frame buffer in sequence by the cache manger and is sent with the audio data sent by the video and audio preprocessor to the multiplex framer. Furthermore, the video frame obtained from the video frame buffer is controlled so as to ensure that the sum of the capacities of the obtained video frame and the audio data sent by the video and audio preprocessor in the multiplex frame is less than or equal to the capacity for containing the video and audio data in the multiplex frame. By adopting the invention, the quality of the transmitted video when an input code rate exceeds bandwidth which can be provided by a logic channel can be improved.