Multilayer dark mudstone is developed in Jurassic strata of the Muli depression within southern Qilian basin. The evaluation of the source rock of Well DK-3 in the aspects of organic matter abundance,organic matter type, and organic matter maturity indicates that organic matter is abundant, organic matter type is Ⅱ1-Ⅱ2, and organic matter maturity is high. The highest hydrocarbon generation potential layer of the source rock is developed in the depth of 140-350 m, which is black deep lacustrine facies mudstone. The relatively high hydrocarbon generation potential layer is developed in the depth of 630-720 m, which is gray mudstone interlayer from the meandering river. Other layers are low in organic matter abundance and have low hydrocarbon generation potential.