Abstract: In this paper, the Cretaceous source rocks and their hydrocarbon genetic conditions in the Fuxin sag are discussed in details. Based on the characteristics of natural gas and crude oil and their correlation with oil sources,it is believed that there are two kinds of sedimentary environments of source rocks in this area. One is the lacustrine mudstone and the other is coal bearing mudstone of swamp facies. The two kinds of source rocks are different in horizon,but the abundance and types of organic matter show no great differences,and the hydrocarbon generation characteristics are also similar. Both of them are dominated by coal gas,supplemented by oil generation. The characteristics of hydrocarbon generation,organic matter maturity and hydrocarbon source rock distribution patterns control the types and abundance of hydrocarbons. Different types of gas were formed in different thermal evolutionary stages. In the immature stage, coal type of biogenic gas dominated,while in the mature-high mature stage, gas of coal pyroysis dominated with small amount of oil. The results of the study will no doubt be helpful to the resource evaluation and oil and gas exploration in the region.