Firstly,the loess site of seismic subsidence of the Yongdeng earthquake in 1995 is investigated,and based on the PGA and the loess's thickness of the site,the value of loess seismic subsidence is calculated by using the relationship of σ_d versus ε_p of the earthquake site.Secondly,the formation mechamism of loessial seismic subsidence is studied by comparing the micro texture of undisturbed loess with the seismic subsided one.The result shows that the value of loessial seismic subsidence calculated using the test curve is smaller than the value of seismic subsidence caused by the earthquake.When exposed to dynamic stress,the void structure of loess will be destroyed by shear stress and become denser under the gravity force.This is the mechamism of loessial seismic subsidence.