Hypersonic stage separation is considered a crucial and challenging problem that involves complex aerodynamic interference and unsteady aerodynamics, which directly determines the success or failure of the two-stage-to-orbiter (TSTO) vehicle. The longitudinal stage separation (LSS) scheme was proposed to improve the safety of the parallel-staged TSTO stage separation, wherein the orbiter separates along the booster’s upper surface with tiny or even no gaps resulting in weak interference. Therefore, the hypersonic flow past the TSTO vehicle at Mach 7 during the LSS is examined by the free-flight experimental test in the JF-12 shock tunnel and laminar CFD simulation. The high-speed pneumatic ejection to launch vehicle model system incorporated into the shock tunnel is developed to conduct the LSS test within the short test duration. In addition, high-speed visualization and image-processing techniques were used to investigate the separation behavior of TSTO. The flow patterns, unsteady measured wall pressure, and the orbiter’s separating trajectory are studied and compared between experiment and simulation. The results between the experiment and CFD show a good agreement with each other. The LSS flow pattern is governed by the type I and VI weak shock wave-shock wave interaction, and short-term shock reflection occurs between stages when the orbiter’s afterbody separates from the booster’s leading edge. Moreover, the aerodynamic interference of LSS for parallel-staged TSTO is weak due to the tiny interstage gap, and the orbiter separates from the booster successfully.