Abstract: Hydrocarbon source rocks in Cenozoic Yaxicuo Group of the Yanghu terrestrial basin, Qinghai-Tibet Plateau are composed of gray, yellow-gray, gray-black mudstone and gray-green marlite, with the thickness of about 100 meter. According to the field outcrop investigation and laboratory analysis results of hydrocarbon source rocks samples, this paper conducts a preliminary evaluation of hydrocarbon source rocks, which is based on the organic matter abundance, organic matter type, organic matter maturity. The results show that organic matter abundance of hydrocarbon source rocks is low, and average organic carbon content of mudstone and marlite are respectively 0.19% and 0.14%. Gray and yellow-gray mudstones demonstrate non-hydrocarbon source rock to poor hydrocarbon source rock characteristics. Gray-black mudstone belongs to good source rocks. Gray-green marlite reachs the standards of moderate-good source rock. Organic matter of mudstone source rocks are mainly of Ⅱ and Ⅲ type and are in immaturity -low maturity stage，while organic matter of marlite source rocks are mainly Ⅰ-Ⅱ1 type and are in low maturity-maturity stage.