1. A JPEG encoding method, comprising: splitting an image into 8×8 pixel blocks and grouping the 8×8 pixel blocks into a number of minimum coded units according to a format of the image, such that each of the minimum coded units consists of a constant number of 8×8 pixel blocks; scanning the minimum coded units in a raster scan order, by subjecting each of the minimum coded units sequentially to forward discrete cosine transformation for converting the 8×8 pixel blocks of each of the minimum coded units into discrete cosine transformation (DCT) coefficients, quantization of the DCT coefficients according to a quantization table to obtain quantized DCT coefficients, zigzag scanning the quantized DCT coefficients to obtain sequenced DCT coefficients sequenced from a lowest frequency to a highest frequency, and entropy encoding for converting the sequenced DCT coefficients into bitstreams in correspondence with the minimum coded units, wherein the sequenced DCT coefficients include AC coefficients and DC coefficients, and the entropy encoding is accomplished by run-length encoding the AC coefficients while not performing differential pulse-code modulation on the DC coefficients; aligning the bitstreams by byte stuffing to obtain byte-aligned bitstreams in correspondence with the minimum coded units; generating compression data of a special JPEG file from the byte-aligned bitstreams; acquiring, in the raster scan order, positions of the byte-aligned bitstreams in correspondence with the minimum coded units in the compression data of the special JPEG file, wherein a position of a first byte-aligned bitstream is recorded as zero; and encoding the positions by a run-length coding and compressing the encoded positions by a Huffman coding to establish a position table.