Abstract: Ancinet alkaline?lacustrine source rocks have been recently discovered in the Lower Permian Fengcheng formation of the Mahu sag in the northwestern margin of Junggar basin. To improve the theory of hydrocarbon generation for this special type of source rocks, the paper studies the basic characteristics of the alkaline?lacustrine source rocks. The compositions of Fengcheng formation are very complicated and dominated by the mixtures of terrigenous debris, carbonates and volcanic materials, which is called alkaline?lacustrine dolomitic diamictite. The bio?precursor of the Fengcheng source rock is characterized by algae and bacteria, the bacterial activity increases with the increase of alkalinity of depositional environment. As a result, the content of organic matter in altered armorphous bodies increases. The organic matter richness of the Fengcheng source rocks has reached moderate to good quality and the kerogen type of the rock is mainly Type I to II1, indicating good hydrocarbon generation potential. The second member to third member of Fengcheng formation form a complete alkali evolution cycle. The second member of Fengcheng formation has reached the peak of the alkali evolution, during which carbonate content is the highest, the bio?precursors are almost amorphous bodies with great hydrocarbon generating potential. The hydrocarbon generation of the source rock is characterized by multistage and high efficiency，with the hydrocarbon generation ratio being almost two times as much as conventional lacustrine source rocks， which lays a foundation for the formation of the 2 "hundred?mile" oil zones in the northwestern margin of Junggar basin and still indicates great exploration potential in the future