Abstract: According as the density change in operation has a characteristic of expansion by steep lead, and as data analysis of pore pressure at drawdown on the earth dams built by dumping soils into the ponded water suggested as well, the author has proposed that one must adopt the consolidation model of Biot to calculate the pore pressure at drawdown on those dams because it has considered for the deformation of skeleton.According to data of prototype, the author has presented an empirical formula for estimating the coefficient of slide resistance K, the coefficient of consolidation cv and the angle of effective equivalent sliding resistance φ's respectively. They are the three chief mechanical parameters affecting the calculating results.In this paper a simple effective pressure method, i.e. an approach of two dimensional consolidation method has been put forward for consideration.