This is the second report on dynamic characteristics of an underwater large rotational shell. This paper focuses on the seismic response of the shell to horizontal ground motion. The natural modes which were obtained in the previous paper are used to estimate the seismic responses, which is based on the theory of power spectrum. The response of the shell in water is very different from that in air because of the effect of large added mass and the seismic input spectrum, and so this kind of numerical analysis of fluid-structure interaction problem has to be analyzed numerically in advance in the design process. And the features of dynamic characteristics of the half drop shaped shell that authors proposed are discussed herein as well.