The invention discloses a quick size reduction transcoding method based on a region of interest, and belongs to the technical field of digital video transcoding of computers. The quick size reduction transcoding method comprises the following steps of: (1) acquiring encoded information from an input H.264 encoded code stream, wherein the encoded information comprises an inter-frame encoding mode, a motion vector and a discrete cosine conversion coefficient; and performing size reduction on a video of a pixel from a common intermediate format (CIF) into a quarter common intermediate format (QCIF), namely 176 pixels *144 pixels; (2) dividing a motion region and a background region by use of motion edge detection according to an image texture characteristic; and (3) adopting corresponding inter-frame modes for macroblocks positioned in different regions according to the motion characteristics of the marcoblocks, performing motion vector reconstruction on the macroblocks positioned in the motion region according to motion differences of the macroblocks by respectively selecting an adaptive motion vector down-sampling method or a most severe weighing method, performing the motion vector reconstruction on the macroblocks positioned in the background region by adopting an average weighing method, and performing motion search on the macroblocks positioned on the edge of and in a motion target. By utilizing the quick size reduction transcoding method, the quality of a transcoded video can be guaranteed, and the instantaneity of size reduction transcoding is realized.