Abstract: According to fossils, lithology, paleo-tectonic and paleogeomorphological features, we comprehensively studied the sedimentary environment of the Yacheng Formation in the Qiongdongnan Basin. It was found that during the period of the Yacheng Formation, the fossils records in the central depression of the Qiongdongnan Basin are continuous, with high abundance and diversity, indicating a restricted shallow-sea environment in the Changchang Depression. According to the sediment filling features from typical seismic profiles, and based on the fact of coal accumulation in the Yacheng Formation, we proposed in this paper a graben source rock model for the Changchang Depression. There are two kinds of source rocks, namely the source rocks of fan delta, coastal marsh and coastal plain around the depression and the source rocks of shallow sea mud and peat layer in the center of depression. The Yacheng Ⅲ and Yacheng I Formations have large areas of coal deposition, and coal source rock is well developed. There were abundant terrestrial higher plants as organic matters for the balanced shallow sea environment of Yacheng I Formation, and preservation condition was also good, of which both could contribute to the formation of carbonaceous mudstone source rocks.