[Objective]:Qiangtang Basin is a large marine petroliferous basin in the northern part of Qinghai-Tibet Plateau. The study area, Sairen area, is located in the south Qiangtang depression, and the research results of oil and gas geochemistry in this area are few and the research degree is low. [Method]:This paper summarizes the ethnic composition and geochemical characteristics of oil-bearing layer samples in ancient oil reservoirs, and draws a conclusion by combining various biomarker parameters. [Results]:The ancient oil reservoirs in this area are in the low-mature stage, C27 sterane is dominant over C29 sterane, and the tricyclic terpanes are mainly C23 tricyclic terpanes, which also indicates that the parent material is mainly marine organic matter, and some terrestrial organic matter contributes. The oil reservoir has a low ratio of vegetation (Pr/Ph<1), and a certain abundance of gamma paraffin and a high C35/C34 liter hopane value are detected in non-hopane. These parameters indicate that the ancient oil reservoir was formed in a sedimentary environment with deep sedimentary water, relatively strong reducibility and certain salinity. [Conclusion]: The analysis of geochemical characteristics of ancient reservoirs in this area lays the foundation for the study of oil and gas accumulation and the understanding of oil and gas enrichment law, and determines the sedimentary environment and parent material source of ancient