The sedimentary environment and provenance characteristics in the Qiongdongnan Basin during the Oligocene and Miocene periods were analyzed using geochemical and paleoecological methods. The results indicate that, during the Early Oligocene, the basin was prevailed by freshwater to littoral environments for the deposition of section 3 of the Yacheng Formation, then altered to sea-land transitional conditions with sea water deepening gradually for section 2, and finally to gulf to outer shelf environments for section 1 of the Yacheng Formation.