Abstract: Since Triassic, Tianshan tectonic belt was compressed by stresses with strike of north-south to produce a series of intermountain basins, for example, Turpan-Hami basin, Kümüx basin, etc. In these basins the Mesozoic-Cenozoic terrestrial clastic deposits are well-developed and widely distributed, specially the Jurassic deposits. The Jurassic strata, except the Upper Jurassic series being red rocks, are mainly composed by dark rocks. These dark rocks, including gray, black mudstones, shales, carbonaceous mudstones and coal beds are of stronger hydrocarbon-generating capability. To general analysis of various materials including deposits, plant and pollen fossils, etc., shows that during the Early-Middle Jurassic the Tianshan region was a feature of lower mountains-hills on geography and was a humid climatic environment. The palaeoenvironment is an advantage to coal accumulation and deposits of dark mudstone.