Abstract: Objectives The booster engine of an underwater supercavitating vehicle needs to be jettisoned promptly after completing its boosting task. The separation process is influenced by the coupling effect between the supersonic gas jet generated by the main engine and the surrounding water medium. To understand the jettison characteristics of the booster engine, Methods A unsteady numerical calculation model for the jettison process of the underwater booster engine was established based on the VOF (Volume of Fluid) multiphase flow model, combined with the overset grid technique. Focusing on the under-expanded and full-expanded conditions, the influence of pressure pulsations induced by the unsteady flow of the underwater rocket engine tail jet on the jettison process of the booster engine sleeve was systematically studied. Results The main engine jet is accompanied by unsteady phenomena such as necking and bulging. Under the under-expanded condition（Pressure Ratio Pe/Pb=2）, the jet wraps around the tail of the booster engine, generating local high pressure and hindering the separation of the booster engine. In contrast, under full-expanded conditions（Pressure Ratio Pe/Pb=1）, the fully developed jet exhibits enhanced axial thrust, and the synergistic effect between hydrodynamic shear and the gas jet promotes smooth separation of the booster engine. Conclusions The ambient back pressure is identified as a key parameter influencing booster engine separation, as its variation directly determines the jet expansion state and its interaction with the surrounding water, thereby significantly affecting the success of the separation process. The findings provide a theoretical basis for the separation process of the booster engine on underwater supercavitating vehicles.