1. An arithmetic encoding-decoding method for compression of a video image block, the method comprising an encoding process and a decoding process, the encoding process comprising: 1) acquiring an information of an image block to be encoded; 2) extracting an encoding command of a weighted skip model; 3) acquiring an index of a reference frame according to the information of the image block to be encoded and the command of the weighted skip model, the reference frame comprising a prediction block for reconstructing the image block to be encoded; 4) acquiring a context-based adaptive probability model for encoding; and 5) performing arithmetic encoding of the index of the reference frame and writing arithmetic codes into an arithmetically encoded bitstream according to the context-based adaptive probability model for encoding; the decoding process comprising: 6) acquiring the arithmetically encoded bitstream of the index of the reference frame; 7) acquiring a context-based adaptive probability model for decoding, wherein the context-based adaptive probability model for decoding is a statistical result of the greater probability event on each bit of the index of the reference frame; 8) arithmetic decoding the arithmetically encoded bitstream according to the context-based adaptive probability model for decoding corresponding to the bit whereby acquiring a binary value of the bit; 9) acquiring the index of the reference frame according to the binary value of each bit; 10) acquiring the prediction block of the reference frame according to the index of the reference frame; and 11) averaging pixel values of the prediction blocks to obtain a pixel value of a reconstruction block, wherein the reconstruction block is the image block to be encoded.