Abstract: The Wan′an Basin lies in the southwestern South China Sea, southeastern Indosinian block, to the west of the Wan′an Fault. It has been a long term controversy on the activity characteristics of the Wan′an Fault and its effect on the formation and evolution of the Wan′an Basin. In this paper, regional geological settings and geological characteristics of the Wan′an Basin and activity characteristics of the Wan′an strike-slip fault are analyzed. The Wan′an Fault was formed early in Late Mesozoic, and experienced a multi-stage activities later on. The Fault cut through the Basin, during which negative flower structures formed. But the Wan′an Fault only caused partial strata change, the strata away from the strike-slip fault were not affected. Thus the basin has obvious characteristics of a rift basin on continental margin.