Abstract: An equivalent nonlinear visco\|elastic constitutive model of soils is developed by the use of secondary development programming interface function, USER SUB.HYPELA, on the MSC.Marc software platform. A typical soft soil site in Guangzhou is selected for the study. The site specific seismic spectrum is analyzed by a two\|dimensional finite element analysis (FEA). To verify the accuracy and reliability of the suggested constitutive model, the result compares well with the results from SHAKE 91, the widely accepted software for the seismic reaction of overlying soils.