Abstract: The Lagrange dynamic equation of dam-foundation structure system under seismic load was derived from system energy.The nonlinear dynamic model of the system was established by using the chaotic theory,and the chaotic behavior of nonlinear dynamics the system response due to earthquake was discussed and analyzed in the time domain,in the phase space,by the Poincaré map,and with the Liapunov exponent and so on.It was shown that the variation of parameters of foundation soil property,such as the soil density,thickness and so on,would be chaotically related with the response of dam due to the seismic load.