Abstract: Whether the hypersonic flow field structure around a slender body in near space exhibiting asymmetry and unsteady is still a debated issue. Particle tracer technique (plane and stereoscopic) and high precision numerical simulation technology, combined with magnitude analysis method were employed to investigate this problem. The particle image results reveal that there are three layers in the flow field around the hypersonic slender body. Unlike the flow field around a hypersonic flat plate or a subsonic slender body, a special “interface layer” with serious particle accumulation is found between the free flow and the boundary layer. Through numerical simulation and magnitude analysis, it is found that the normal velocity is no longer 0 in this interface layer, and there is a large normal gradient pressure. The presence of leading edge shock and expansion wave as well as the interaction of large curvature surface result in the formation of interface layer. The results also show that asymmetry and unsteady of the flow structure mainly occur in the interface layer at high Reynolds number. The thickness of the interface layer increases with the increase of the angle of attack, and the deformation of the interface layer results in the asymmetric and unsteady of the fluid field at large angle of attack. These findings lay an analytical foundation for the aerodynamic performance and control of hypersonic missiles in near space.