Abstract: The distribution of sedimentary facies belt in Chang-3 member of Yanchang formation is the key factor to control the distribution of tight sandstone reservoir. Based on the analysis of core, drilling, logging and seismic data, combined with the sedimentary background of Yanchang formation in Ordos basin, it is considered that the braided river delta and shore shallow lacustrine sedimentary system are mainly developed in Xun-Yi block area of Chang 3 member. The sedimentary microfacies such as underwater distributary channel, distributary bay and mouth bar are identified. Among them, the underwater distributary channel has wide development scale, large sand body thickness (single sand body thickness of 8.0～18.0 m), and good physical properties (average porosity of 11.3%, average permeability of 0.7×10^–3μm²), which plays an important role in controlling the enrichment degree of oil and gas, and are favorable exploration facies zones.