Three-dimensional calculation model of Shanghai soft soil is built, in which the Davidenkov model is proposed to describe non-linear dynamic characterisric of the soft soil based on the testing data. The rules of seismic response are studied. It is showed from calculation that： （ 1 ） the content of high frequcy in seismic wave is decreased and low frequcy part is increased from the bottom of the soft soil to its top ; （2） the seismic response is not always been enlarged, its influence lies on the characters of soft soil and seismic waves ; （ 3 ） it is feasible to simplify the seismic response analysis of free-field as plain strain problem; （4） the calculation curves and testing curves between dynamic shear modulus and dynamic shear strain are agree well with each other, which testified that it is ratonal to consider the non-linear characteristic of soft soil by the Davidenkov model. The study results can provide reference for engineering seismic design in soft soil area.