Abstract: Whether the hypersonic flow field structure around a slender body in near space exhibits asymmetry and unsteadiness remains a topic of debate. To investigate this, particle tracer technique (both plane and stereoscopic), high-precision numerical simulations, and magnitude analysis methods were employed. The particle images of the flow field structure of a slender body in its longitudinal section and multiple spanwise cross-sections were obtained in the wind tunnel experiments. The particle image results reveal a three-layer structure within the flow field around the hypersonic slender body. Unlike flow fields around hypersonic flat plate or subsonic slender bodies, a distinct “interface layer” with significant particle accumulation is observed between the free flow and the boundary layer. Numerical simulations and magnitude analysis indicate that the normal velocity in this interface layer deviates from zero, accompanied by pressure peak value and a large normal pressure gradient, which is caused by the interaction of the leading-edge shock, expansion wave and large surface curvature. The results also show that asymmetry and unsteadiness in the flow structure primarily occur within the interface layer at high Reynolds number. The thickness of the interface layer increases with the angles of attack. When the angle of attack is large and the Reynolds number increases, the interface layer undergoes distortion and deformation, which leads to asymmetric and unsteady characteristics in the flow field.. These insights provide an analytical foundation for understanding the aerodynamic performance and control of hypersonic missiles in near space.