Abstract: The petrographic associations and sedimentary environments are treated for the B Formation of the Upper Carboniferous Qiati'er Group in the Tianshuihai-Mazar block, western Kunlun Mountains in terms of sedimentary structures, thin section and grain-size analysis. The lithology in the Formation consists of fine-grained sandstone, siltstone, mudstone and shale, which are involved into a succession of lacustrine-continental delta deposits. The delta facies may be subdivided into the delta plain, delta front and prodelta subfacies. The detritus come from the recycled orogen and mixed source areas. The results of research in this study will provide important information for better understanding of the tectonic evolution of western Qinghai-Xizang Plateau.