Abstract: The appropriate system parameters include the equivalent natural period and the effective damping ratio, which are crucial to ensure a reliable estimation of the maximum inelastic displacement response by the equivalent linearization method. A standard particle swarm optimization algorithm is proposed to determine the equivalent system parameters through a search in the two-dimensional space of the period and damping ratio. The experiments are conducted to investigate the validity and efficiency of the proposed algorithm. Analysis indicates that the proposed algorithm can obtain system parameters with the same precision compared with the statistical method while its solution efficiency is about 2.0 percent of the statistical one. The defect of huge amount of calculation of the statistical method to obtain the system parameters is thus overcome. As an application, the equations of the equivalent system parameters of the elastic-plastic SDOF systems are deduced using the proposed algorithm with the assumption of constant displacement ductility ratio.