In order to study the mutual effect of soil weight and underground structure under earthquake, the effecting rules of gravity on soil properties are summarized, and then mutual effect of gravity and seismic response, treatment of crustal stress and way of adding gravity are studied. Finally, the applicability of adding gravity, effecting extent of gravity on seismic response are evaluated through typical computational examples. The results show that:(1)Under earthquake, the gravity of underground structure and seismic response effect mutually. (2)In numerical calculation,gravity is added by the form of loading time history. (3)When the soil is nonlinear medium, calculation exists error by using superposition principle. And the errors become larger along with the increment of soil nonlinear extent, but not monotone increase, which is affected by various factors such as seismic wave, engineering site and so on.