Abstract: During spacecraft attitude maneuvers, rendezvous and docking, and orbital transfer, the combined effects of the microgravity environment and translational and rotational disturbances induce significant propellant sloshing in partially filled tanks. The resulting slosh forces interact with the spacecraft, giving rise to rigid–liquid coupled dynamics that degrade engine performance and attitude control accuracy. In this study, a six-degree-of-freedom rigid body-liquid coupled sloshing solver for spacecraft propellant tank systems was developed based on the open-source computational fluid dynamics platform OpenFOAM. Numerical simulations were performed to systematically investigate the effects of the initial angular velocity, external acceleration loading history, and fluid-to-rigid-body mass ratio on the dynamic characteristics of the coupled system. The results show that rigid–liquid coupling not only introduces additional disturbances into the rigid-body motion but also increases the tendency of dynamic instability. Meanwhile, viscous dissipation induced by propellant sloshing accelerates the attenuation of fluid oscillations compared with the uncoupled case. Furthermore, the coupling effect causes the system response to deviate from the desired motion, revealing a trade-off between energy dissipation and system robustness. These findings demonstrate that rigid–liquid coupling and its dependence on operating parameters should be fully considered in the design and stability assessment of spacecraft attitude control systems.