The drawdown velocity of foundation pit water level in existing projects is mostly determined empirically, and the influence of water level drawdown velocity on the seepage field and stress field in the weir body is not fully considered. The water level change of the foundation pit is an important factor affecting the stability of the cofferdam. Therefore, it is of great practical significance to study the variation law of seepage field and stress-strain field of cofferdam during the pumping process of foundation pit, to accurately judge the stability of cofferdam slope and to formulate reasonable pumping and drainage scheme. Based on the 1th temporary earth-rock cofferdam of Yongning Water Conservancy Project, the nonlinear finite element method is used to calculate the spatial-temporal evolution of seepage field and stress-strain field of earth-rock cofferdam under different drawdown velocity of water level. The overall deformation of the cofferdam, the shear failure and instability characteristics of the slope, and the influence of water level changes on the stability of the cofferdam slope are analysed. The results show that the distribution of unsaturated zone and negative pore pressure zone is basically the same. The excessive water level drawdown velocity is easy to break the balance between the seepage field and the water level fall time, resulting in the rise of saturation line height and the increase of the upper bending rate. The effect of seepage control is weakened, and the stability of the slope is poor. The shear deformation of the cofferdam slope develops from the foot of the slope and extends to the crest of the cofferdam until it runs through. At the same time, local plastic deformation occurs at the foot of the outer stone slag berm, and the maximum deformation transits to the foot of the inner slope. Under the action of seepage force, the stability of the inner slope is always smaller than that of the outer one; the change of water level drawdown velocity is very sensitive to the stability of the slope, when the drawdown velocity is greater than the critical velocity, the stability of the slope decreases sharply. Combined with the characteristics of earth-rock cofferdam, on the basis of satisfying the stability of cofferdam, the drawdown velocity of water level should be controlled within 1.28m/d. The research results of the thesis have reference significance for the selection of pumping speed of foundation pit similar to earth-rock cofferdam.