|Abstract:Paleo-sedimentary environment of Chang 4+5 Member of Upper Triassic Yanchang Formation in Ordos Basin, including the paleosalinity, paleotemperature, Paleobathymetric, paleoclimate and paleoredox conditions were restored through geochemical element analysis of 47 mudstone samples collected from the outcrop sections around and wells drilled in the basin and using a series of identification indexes of paleo-climate, paleo-salinity and paleo-redox conditions, such as Sr/Ba, Th/U, Rb/K2O, boron (B) method, cobalt (Co) method, U, V/(V+Ni), V/Cr, Ni /Co, Ce/Ce*, Eu/Eu*, Sr/Cu, Mg/Ca and other element ratios. Comprehensive analysis shows that in sedimentary period of the Chang 4+5 member, the paleo-temperature ranged from 14.5 ℃ to 24.6 ℃, with a semi-humid-semi-dry temperate subtropical climate. the paleosalinity was continental brackish water environment, between 0.49 ‰~3.7 ‰. the paleo water-temperature was about 8.9~11.9 ℃, the palaeo water-depth ranged from 10 ~85 m, and the sediments were formed under weak reduction-oxidation conditions. Compared with the sedimentary period of Chang 7 member, although dark mudstone is developed in Chang 4+ 5 member, due to the obvious change of environmental conditions, the increase of air temperature leads to the increase of water temperature and salinity, which ultimately affects the development and preservation of organic matter and is not conducive to the formation of effective source rocks in this period.