Upper Triassic Yanchang Formation in Ordos Basin is characterized by deep thickness of sedimentation and abundant oil and gas resources, which is one of the most important target zones for oil and gas exploration in the basin. According to the study of geologic background and reservoir characteristics of Chang 9 oil reservoir set in Hongjingzi area, this paper comprehensively analyzed the sedimentary facies, sand body distribution, hydrocarbon migration pathway and paths. The result shows that the Chang 9 oil reservoir set in the study area is typical shallow water delta sedimentary system, with Chang 7 organic material as the main oil sources. The excess fluid pressure is the main driving force for oil and gas generated by Chang 7 hydrocarbon source rocks, which migrated to the top structure of Chang 9 reservoir through the pores and fractures and accumulated. The main controlling factors of hydrocarbon accumulation are oil source, channel and pressure difference.