Sloshing response of a cylindrical liquid storage tank with the double deck type floating roof (DDFR) subjected to seismic excitation is considered in this paper. The aim of the paper is to clarify the significant parameters that should be considered in the seismic design of a DDFR and proposing a practical seismic design procedure for evaluating the dynamic stresses inside a DDFR. A numerical method including fluid–structure interaction and the geometry details of a DDFR tank are established. The geometric nonlinear effects on the seismic behavior of the DDFR as well as the accuracy of common analytical solution suggested in the literature are examined by the numerical model. The numerical results show that the geometric nonlinear effects can considerably reduce the seismic stress in DDFR, but have no significant effect on the liquid hydrodynamic pressure exerted on the DDFR and the roof's vertical displacement. It is also revealed that not only the general displacement of DDFR but also the local effects of liquid hydrodynamic pressure on the bottom plate should be considered for seismic design of a DDFR. Finally, a design procedure for the evaluation of dynamic stress in the DDFR due to the seismic loads is proposed and discussed.