In order to study the characteristics and controlling factors of the reservoir rocks in the 4th member of the Xujiahe Formation in the Western Sichuan，the methods of outcrop and core observation，thin section analysis，porosity and permeability test，CL，SEM and XRD were used. The results show that the main rock lithology of the fourth member of Xujiahe Formation in the study area is medium-grained lithic sandstone with good sorting and medium or poor grinding roundness. The cementation types are mainly pore-filling type and compression type. The average porosity is 6.22%. The average permeability is 0.279 mD. The secondary pores consist of intergranular dissolved pores and intragranular dissolved pores，which are the main reservoir space. The relatively high-quality reservoirs are controlled by sedimentary facies belts，clastic components，rock structure and diagenesis. The subaqueous distributary channel，the estuary dam of delta front and the superimposed distributary channel of delta plain are favorable sedimentary microfacies. Medium-grained and well-sorted lithic sandstone with high quartz content and low lithic content has the best reservoir performance. Secondary pores generated by feldspar and debris dissolution can effectively increase the reservoir space，which accounts for about 85% of the total porosity and is the key to the formation of high quality reservoirs. Fractures can increase permeability to a certain extent and play an active role in improving storage capacity of reservoirs.