Abstract: During the main engine cutoff of a liquid launch vehicle, the axial acceleration inside the propellant tank rapidly decays to zero, which may lead to a significant amplification of propellant sloshing and consequently exert a pronounced impact on the vehicle attitude dynamics. In this work, a three-dimensional numerical model is established to investigate propellant sloshing in a tank under a sudden drop in axial acceleration, with a systematic analysis of the flow responses associated with different initial sloshing phases. The results indicate that after the rapid decay of axial acceleration, the propellant climbs rapidly along the tank wall due to inertial effects and may even surge toward the top of the tank. Moreover, the initial sloshing phase is found to have a direct influence on the subsequent sloshing response. The residual sloshing kinetic energy at the instant when the axial acceleration decreases to zero is identified as the key factor governing the subsequent propellant flow pattern. An optimal engine cutoff phase is shown to exist, at which the sloshing kinetic energy entering the microgravity coasting phase is minimized. The present results provide useful references for engine cutoff timing design and the optimization of propellant management strategies.