Abstract: The generation arid diffusion processes of pore pressure in saturated Sand layer underlying the Douhe River earth dam under different drainage conditions during the 1976 Tangshan earthquake as well as redistribution and dissipation processes of pore pressure soon after the earthquake are analysed numerically using finite element method in this paper. Expressions of total residual pore pressure and critical ratio of pore pressure in different failure stages of liquefaction in saturated sand are deduced based on the dynamic tri-axial tests. Criteria for evaluating liquefaction potential are defined and then the seismic stability of Douhe River dam during the 1976 Tangshan earthquake is analysed. The damage to Douhe River dam can be explained better by the results obtained, and it is found that sand gravel wells and sand gravel layers play an obvious role in drainage of the whole system.