Abstract: The irregular features of the boundary were not adequately utilized in current compression chain code methods.Aimed at this,a lossless combined compressed chain code which compresses the irregular line segments and is expressed as combined code was proposed.First of all,without affecting the boundary representation,redundant boundary grids of which the directional difference is 135°or orthogonal 90°were removed,so that the distribution of directional difference was more concentrated;secondly,run-length encoding to compress to the regular line segments was extended to irregular line segments,and the repeated combination pattern of±45°followed by some 0°can be expressed by three parameters,so as to further improve the compression rate;finally,by comparing the frequency and compression rate of different combinations,a combined chain code of which the structure was“identify code+distinguish code+length code”was designed.The experiments were operated on 8 provinces’rasterized borders,the results show that the compression ratio of the proposed method can reach 35.39%on average,and it is improved by more than 20%compared with the original directional difference chain code.