Lateral jet is a common scheme for separation of aircraft at high altitude. High-pressure jet from the after-body engine will have a significant impact on the flow around the forebody, which will bring qualitative changes to the aerodynamic characteristics of the forebody and pose great challenges to the design of the control system. By constructing an interstage separation model with lateral jet, the interstage separation flow field under different inflow conditions, jet conditions and separation spacings was numerically simulated, and the influence of lateral jet on the aerodynamic characteristics of the forebody was systematically studied. The results show that the forebody is mainly affected by the high-pressure backflow zone at small separation spacings, and is mainly affected by the separation vortex in the gap at large separation spacings; When the separation vortex at the bottom of the current body separates from the high-pressure backflow zone, the forebody is no longer affected by the afterbody; When the total jet pressure increases or the Mach number decreases, the influence of separation spacing on the aerodynamic characteristics of the forebody increases; When the total jet pressure or Mach number increases, the spacing needed for the disappearance of the influence of the afterbody jet on the forebody increases.