Abstract: A wide variety of liquid storage tanks are designed for nuclear facility. The sloshing analysis is an essential part in seismic analysis of liquid storage tanks. Unlike single-directional mass-spring model adding hydraulic loads over the bottom to those on the walls, a combined mass-spring model was proposed which separated the hydraulic loads on the side walls from those over the bottom and the formulas for rectangular containers were given. The combined model was convenient to build up in 3D FEM and produced more reasonable stress distribution over the bottom. According to numerical results of sloshing analysis for CEFR sodium storage tank, the combined model is reliable. The combined mass-spring model provides a new technique in seismic analysis of liquid storage tanks.