Abstract: The Wan'an Basin is located on the southwestern margin of the South China Sea, and its unique tectonic position has made its filling evolution an important window to study the expansion of the southwest submarine basin and the strike-slip faults on the western margin. Based on the results of core analysis, correlation of core and seismic reflections, and seismic facies, the characteristics of the infilling of the Wan'an Basin and the sedimentary responses to tectonic events in Miocene were studied. Four main sedimentary facies related to hydrocarbon reservoirs have been developed in the basin during Miocene, including delta, turbidite fan, carbonate platform and reef. In the Early Miocene, delta facies was widespread in the study area. During the Middle Miocene, delta facies developed only in the western part of the Wan'an Basin, and carbonate platform deposits began to develop in the eastern part. And turbidite fan, sourced from the western part, began to develop in the Late Miocene. The southwestern sub-basin of the South China Sea stopped expanding at the end of the Early Miocene and the strike-slip fault activities on the western margin of the South China Sea at the end of the Middle Miocene formed two important tectonic unconformity interfaces, which played an important role in constraining the Miocene sedimentary filling in the Wan'an Basin.