The overtopping dam-break is a process of gradual development with soil-water coupling. It relates to the comprehensive effect of active erosion force of flow and passive resistance of dam body material. Based on the forward theory of current world dam break and refered to the previous field prototype test results, the paper made a macro stage division for dam break process and carried out a special flume experiment by using PIV technology. It exhaustively investigated the fine structure of surface flow and headcut flow. Through the experiment, the paper got the geometric and hydraulic characteristics of two typical flow and summarized the erosion mechanism of each flow structure on dam body. The result can make us understand the dam-break mechanism inessence and also provide technical reference for numerical simulation and other research work on dam break.