Dynamic characteristics of a fluid-structure coupling system with variable mass in a tank of spacecraft were mainly investigated. Dynamic model of system was established by using boundary element method and finite element method based on virtual mass method (VMM). The effect of variable mass on the dynamic response of the system was mainly considered. The vibration responses of the system with variable mass tank were calculated by using Newmark direct integration method. Results show that the decrease of the mass of system induces the increase of the vibration frequencies of the system

and generates an additional negative damping. The frequency range can be determined by the range of the mass of the system. It is derived that the additional negative damping induced by variable mass is proportional to rate of change of the mass. For lateral vibration of the system

the additional negative damping effects the vibration stably in the whole process

while for longitudinal vibration of the system

the influence of additional negative damping increases with time prolonged.