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) inputting an image block to be encoded into an encoder, wherein the encoder comprises a first module for acquiring information of an image block, a second module for extracting an encoding mode, a third module for acquiring an index of a reference frame, a fourth module for acquiring a probability model for encoding, and a fifth module for arithmetic encoding; 2) activating the first module to acquire information of the image block to be encoded and then to transmit the information of the image block to be encoded to the third module; 3) activating the second module to extract an encoding command of a weighted skip model and then to transmit the encoding command of the weighted skip model to the third module; 4) activating the third module to acquire an index of a reference frame according to the information of the image block to be encoded and the encoding command of the weighted skip model, the reference frame comprising a prediction block for reconstructing the image block to be encoded, and then to transmit the index of the reference frame to the fifth module; 5) activating the fourth module to acquire a context-based adaptive probability model for encoding and then to transmit the context-based adaptive probability model for encoding to the fifth module; and 6) activating the fifth module to perform 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: 7) transmitting the arithmetically encoded bitstream to a decoder, wherein the decoder comprises a sixth module for acquiring the arithmetically encoded bitstream, a seventh module for acquiring a probability model for decoding, an eighth module for arithmetic decoding, a ninth module for producing the index of the reference frame, a tenth module for acquiring the prediction block, and an eleventh module for calculating a reconstruction block; 8) activating the sixth module to receive the arithmetically encoded bitstream and then to transmit the arithmetically encoded bitstream to the eighth module; 9) activating the seventh module to acquire a context-based adaptive probability model for decoding and then to transmit the context-based adaptive probability model for decoding to the eighth module, 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; 10) activating the eighth module to perform arithmetic decoding of the arithmetically encoded bitstream according to the context-based adaptive probability model for decoding corresponding to each bit of the index of the reference frame whereby acquiring a binary value of each bit of the reference frame, and then to transmit the binary value of each bit of the index of the reference frame to the ninth module; 11) activating the ninth module to acquire the index of the reference frame according to the binary value of each bit of the index of the reference frame and then to transmit the index of the reference frame to the tenth module; 12) activating the tenth module to acquire the prediction block of the reference frame according to the index of the reference frame and then to transmit the prediction block of the reference frame to the eleventh module; and 13) activating the eleventh module to average 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; wherein: in the encoder, the first module and the second module are physically connected to the third module; the third module is physically connected to the fourth module; and the fourth module is physically connected to the fifth module; and in the decoder, the sixth module and the seventh module are physically connected to the eighth module; the eighth module is physically connected to the ninth module; the ninth module is physically connected to the tenth module; and the tenth module is physically connected to the eleventh module.