In order to improve the performance of the rocket-based combined-cycle engine, the influence of the rocket exit area under the ejector mode was studied. Through numerical simulation research, the mass flow of captured air was mainly affected by the entrainment performance under the condition of low flight Mach number. The larger the rocket exit area, the better the entrainment performance. However, with the increase of flight Mach number, the kinetic energy of the entrainment air increases, and there is flow choking in the isolator. The mass flow of captured air was mainly limited by the geometric size of the isolator, independent of the rocket exit area. Under subsonic conditions, the smaller the rocket exit area is, the lower the specific impulse of the engine is, and when the exit dimensionless area is 3.15, the rocket plume will expand and impact the wall, which can cause a sudden reduction in performance. Under supersonic conditions, the smaller the rocket exit area, the higher pressure in the combustor, and the better performance of the rocket-based combined cycle engine.