A set of clastic rocks with thickness of more than one thousand meter was deposited in the Lower Cretaceous in front of the Kunlun Mountain in southwestern Tarim Basin. They are mainly red fine-middle grained sandstone, maroon sandy conglomerate, conglomerate, with mudstone and thin interbeded grey fine sandstone. Based on the group of fossils and lithologic characteristics, the Kezilesu Group are divided into 5 lithologic members: the first one is the sage green mudstone Member of which the sedimentary environment was mostly lake and fan delta. The second one is sandy conglomerate of which the sedimentary environment was mainly the front and plain of fan-delta. The third one is thick gravel-sandstone of which the sedimentary environment was mainly braided river and braided river delta plain. The fourth is mostly fine-grained sandstone interbedded with mudstone, of which the sedimentary environment is underwater distributary channel on braided river delta front. The last one is mudstone interbedded with sandstone of which the sedimentary environment is braided river delta front, probraided river delta and shore-shallow lake. The evolution of sedimentary environments changing from quickly deposited fan-delta to alluvial fan-braided river to braided river delta shows the significant development period of the fault basin. Over 70 percent of the clastic rocks is medium-fine-grained sandstone, whose pore types are original intergranular pores with porosity of 10%~20% and permeability of 1~1000×10－3μm2. The Kezilesu Group is the important strata for oil and gas exploration in the Mesozoic in Tarim basin.