The effect of soft soil layer on site response has long been of great interest.This paper presents studies on the nonlinear response of a deposit containing a soft layer

and investigates the influence of the geometrics properties and shear wave velocity of the soft layer on the spectra of ground surface motions.Results indicate that:（1）A soft soil layer existing in a deposit will push the characteristic period of ground acceleration spectra to long time domain and cause multipeak spectra at long period domain

and usually the difference ratio between the spectra with/out soft layer exceeds far the acceptable range in engineering.（2）The maximum difference ratio of spectra（MDRS）caused by a soft soil layer increases approximately linearly with the thickness and overlaying depth of the soft layer

and increases nonlinearly with the decreasing of shear wave velocity of soft layer.In the calculation case of this paper

the value of MDRS approximates to 12.（3）The sensitive period which makes the MDRS increases with the thickness and overlaying depth of the soft layer as well as with the input earthquake amplitude

and increases with the shear wave velocity of soft layer decreasing.Mechanical analysis indicates that the too large shear strain in the soft layer relative to nearby normal site layers maybe the main reason which causes the ground surface spectra change obviously.