Abstract: Abstract：The Ordovician Yinshan Formation in Tarim basin has been regarded as the focus layer for breakthrough in oil and gas exploration. Through comprehensive use of cores, slices, logging, analytical and test data and three-dimensional seismic data, the authors analyzed the reservoir characteristics and influencing factors of Yinshan Formation. Some conclusions have been reached： the lithology of Yingshan Formation is generally composed of fine-grained dolomite,calcarenite and limy dolomite, whereas the reservoir space comprises mainly dissolved pores and cracks; the top of the Formation is the weathered crust reservoir with the development of dissolved pores and cracks, while the lower part of the Formation is the fractured reservoir, whose space is mainly cracks; in vertical direction, the upper part is mainly composed of pores and cracks, whereas the lower part comprises mainly cracks with strong heterogeneous nature; the tectonic unconformity formed in the first episode of the middle Caledonian tectonic movement seems to have been the important factor responsible for reservoir development; the faults and tectonic fractures formed during the middle Caledonian tectonic movement and the Early Sea West tectonic movement were the key factors for the reservoir development; during the early Permian period, the karst baking associated with volcanism and the burial dissolution exerted a positive effect on the reservoir development