In order to reveal the influence of dynamicand P －Δ and P －δ effects on the single-degree-of-freedom system ( SDOFS) ,the theoretical simulation and the numerical calculation are both conducted from the viewpoint of "stiffness reduction”. First,the formulae of the coefficient of the lateral stiffness reduction precisely considering second-order effects are deduced,which regard the influence of the second-order effects on the structural dynamic
response as the lateral stiffness reduction. Next,the theoretical formula of the response spectrum method precisely considering the second-order effects of SDOFS is proposed and is simplified based on the standard response spectrum of the seismic design code. Then,the differences between the P －Δ and P －δ effects and the P －Δ effect are examined under the static and the dynamic conditions. Finally,the proposed method is verified by the results of the finite element numerical calculation,finding that the results of the proposed method accord well with the numerical ones.