In this paper, the problem and analysis method of underwater storage tanks resting on a horizontal seabed is presented under stochastic earthquake loading. The tank is axisymmetrical and has a flexible wall/roof. The finite element method is used for the response solution. A solid axisymmetrical finite element has been formulated to idealize the tank whereas an axisymmetrical fluid element is used for the idealization of the fluid domain. The Eulerian formulation of the fluid system is used to calculate the interactive water pressure acting on the tank during the free motion of the tank and earthquake motion. For the response calculation, the modal analysis technique is used with a special algorithm to obtain natural frequencies of the water-structure coupled system. For the stochastic description of the earthquake loading, the modified Kanai–Tajimi earthquake spectrum is used. Finally, the analysis method presented in the paper is demonstrated by an example.