Abstract: The lithology of marine-continental transitional coal source rocks is complex and changeable, and the maturity recovery of oil and gas is difficult. Taking an oil and gas field of Xihu Sag as an example, the recovery method of oil and gas maturity is established by means of vitrinite reflectance of source rock, chromatographic mass spectrometry of saturated hydrocarbon and aromatic hydrocarbon and carbon isotope analysis of thermally simulated natural gas. The maturity of oil and gas is calculated and the source of oil and gas is clarified. The results show that oil maturity is mainly distributed between 0.8% and 1.0%, which is mature oil. The maturity of natural gas is mainly distributed between 1.0% and 2.0%, belonging to mature-high mature gas. The maturity of oil and natural gas in the same interval is different, which reveals the difference in the source and accumulation process of oil and natural gas. Based on the thermal evolution history of source rocks, it is considered that the oil in this area mainly comes from the slope sub-depression, and the ring sub-depression zone is the favorable exploration area. Natural gas mainly comes from the main depression of the sag, and the slope zone is a favorable zone for natural gas, with great exploration potential.