Abstract: In this paper,the arrangement of PIV system and the experiment scheme in D4 wind tunnel in BUAA were presented.The application of PIV technology in the investigation of asymmetric vortices structure over forebody was also presented.The flow structures of sections X/D=2 and 3.35 were investigated at 50° angle of attack and Re=0.14×10~6~0.55×10~6.Experiment results showed that with the increase of Reynolds number the flow structures on the sections had the trend of developing from two vortices to three vortices.In the sub-critical regime,the vortices had the main effect on the asymmetry of pressure distribution.In the initial critical and critical regime,the flow condition and the separation shape of the boundary layer had the main effect on the asymmetry of pressure distribution.In the initial critical regime,the trend of developing from two vortices to three vortices along the model axis happened more upriver than the trend in the sub-critical regime.