Keywords: Equivalent spring element method; Additional mass method; Dynamic characteristics Abstract. In order to study the effect of fluid on the structural dynamic characteristics, in this paper, a three-dimensional spring element was applied to the dynamic analysis of the fluid. Based on the simplified model of nuclear power structure, the upper water tank of concrete containment was analyzed in different water level by equivalent spring element method and additional mass method. The theory of three-dimensional equivalent spring element was put forward, and then the numerical simulation of different water tank was carried out. The computational results of the two methods showed that the equivalent spring element method and the additional mass method have the similar accuracy.