Abstract: PCCS in AP1000 includes two parts, namely a reinforced concrete containment, which is modeled as structural dynamic, and a gravity drain water tank, which is modeled as lumped masses known as sloshing mass, impulsive mass and rigid mass. Based on the seismic response characteristics of the water storage tank and the reinforced concrete containment, the conventional model and isolated model are built respectively. The optimal analysis of isolated layer parameters, namely the natural period and damping, are provided. Using the nuclear power plant dynamic characteristics and structural design codes, three models, namely conventional structure, isolated model on design code, and isolated model on the optimal parameter, are studied in MATLAB software. The seismic responses of all models are compared under the real earthquake ground motions. Results show that the isolated model with the optimal parameter provided by this paper has the best seismic function in all models.