There are low-richness organic matter and high hydrocarbon translation ratio of Tertiary hydrocarbon source rocks in Qaidarn basin. It was formed in environment of salt water and deoxidization. It's characteristic differ from eastern petroliferous basin of China. Especially, nearly 80 percent of the proved oil resources in Qaidam Basin are immature oil. In order to explain this anomalous feature, we have improved the traditional method of kerogan extraction and combined the determination of kerogan type with maceral analysis of thin-sec-tions of mudstone. Writers have identified Botryococoous and Coccolithophorida, which are enriched fatty acid. According to our research, it is concluded that Botryococoous and Coccolithophorida are the real organic matter that forms the immature oil deposits in the Qaidam Basin.