Abstract: Based on the lithologic features, precipitation facie and organic geochemistry characteristics of dark mudstone of Middle Permian Zhesi Formation, the authors summarize organic matter abundance, kerogen type and thermal evolution characteristics. The total thickness of dark mudstone is 640m. The content of TOC is between 0.38-1.78%, and samples of TOC values more than 0.6% account for 98%. The kerogen type is dominated by Ⅱ-Ⅲ type, Tmax values are 354-548 ℃, and the average vitrinite reflectance is 3.28%, which shows that kerogen has turned into high-extra evolution stage.In the study we also found that S1+S2(<3.2mg/g) and IH(<16mg/g·c) are very low. The reason is generating rock of weathering, has lived through the thermal evolution and hydrocarbon expulsion. The hydrocarbon expulsion happened in Early Permian and Late Cretaceous.