Abstract：Though the analysis of clay minerals, major and trace elements of the Carboniferous mudstone samples collected from cores in Bachu, Tazhong and Tahe areas and field outcrops around Xiaohaizi in Bachu and Sishichang in Akesu, palaeosalinity character of water environment when this mudstone was deposited in these regions was revealed by quantitative semiquantitative analysis involving calculation using Adams and Couch palaeosalinity formula, equivalent boron determinate method suggested by Walker and strontiumbarium ratio method. The results indicate the range of palaeosalnity is 4.7‰~33.9‰, and display that the majority of mudstone samples show semisaltwater sedimentary environment and freshwater brackish environment is secondary while saltwater environment is less. In plane view, from northeast of the basin to southwest, the palaeosalinity concentration of mudstone in Mudstone Section, Bachu Formation increased in a general view. Mudstone in Sandstone and Mudstone Section, Kalashayi Formation shows the minimal palaeosalinity concentration in the center and the maximal in the southwest part of the basin, and in the north part of the basin, the palaeosalinity concentration is moderate. Owing to palaeosalinity information extracted form mudstone is one of the indications to determine the sedimentary environment, it can provide important reference in aspect of water character for identifying controversial sedimentary environment of Sandstone and Mudstone Section in Kalashayi Formation and Breccia Section in Bachu Formation. Palaeosalinity of water environment when Carboniferous mudstone was deposited has a good correlation with eustacy and sedimentary facies evolution in this period, and these mudstone was formed in sedimentary environment in which sea level is relatively in fall or slow rise and palaeosalinity is relatively low.