By using PIC method ,the vertical impact on the water surface for a 2-D su-personic flat-head body with a detached shock wave is studied in this paper.Numerical resultsshow that various complicated flow phenomena such as the supersonic flow around the body,theair cushion effect,the propagation of the shock wave in water,the initial formation of the wa-ter-entry cavity and the pile-up of the water surface are simulated successfully.