Sloshing refers to the movement of a liquid free surface caused by disturbances of a liquid partially filled in a container. Sloshing generates forces and moments owing to the pressure distribution on the liquid free surface, which affect the dynamic stability of the vessel. An equivalent mechanical model that exerts the same amount of force and moment on the vessel wall as sloshing is used to analyze the sloshing effect on the vessel. This phenomenon also occurs in the fuel tank of a satellite structure that controls the attitude using a propulsion system with a liquid propellant. A satellite with a deployable synthetic-aperture radar (SAR) antenna, which is made of a thinner and lighter material, is likely to be more susceptible to various disturbances, including liquid sloshing. In this study, an analysis of a deployable SAR antenna reflector vibrating from the liquid fuel sloshing between the changes in the satellite’s attitude was conducted. The equivalent mechanical model is used to simulate the sloshing effect, and the vibrations of the reflector are compared and analyzed with and without the sloshing effect for the specified torque input.