Abstract: Be aimed at oil filling conditions in reservoir and taking the Ordovician reservoir in Tahe Oilfield as an example, the authors attempt to conduct an artificial partitioning experiment of mixed crude oils by double filling into the reservoir using individual filling oils as the end-member of artificial mixed oil samples. By mains of data processing soft-ware in the GC working station, the GC peak area ratios R/U between resolved components and unresolved complex mixture (UCM) can be measured on the gas chromatograms of all artificial mixed oil samples, the conversion diagram of double filling ratios of subsurface mixed crude oils were established based on the correlation curve between the partitioning ratios of artificial mixed oil samples and their R/U values. Owing to the heavy biodegradation only on the early filled oil, the R/U ratios can also be measured on the GC of subsurface crude oil samples, which can characterize the double filling ratios of hydrocarbons in the mixed crude oils, and then the double filling ratios of subsurface crude oils were obtained from above-mentioned conversion diagram and the R/U values.