The present invention relates to security video monitoring for and discloses a motion analysis method and a code stream conversion method based on video compression code stream and apparatus thereof. In the invention, a total number of bits or a residual bit number of a macro block in a video I frame is extracted from the video compression code stream; weighted average is performed to the total number of bits or a residual bit number of the macro block whose position corresponds with the current macro block in the latest M frames and a background value is obtained through calculating; a difference value between the total number of bits or the residual bit number of the current macro block and the background value is calculated, and if the difference value is greater than a predetermined first threshold, then a moving object is determined to be detected. Weighted average background modeling is performed to macro block information, such as the total numbers of bits or the residual bit number of the macro block corresponds with the current macro block in latest M frames. Threshold determination is performed to a background difference value matching result and motion analysis is performed. Coding picture of the macro block between the frames does not need to be calculated. Calculated quantity is small and the adaptability to the code stream and a scene is strong.