Foundation dewatering combined with a waterproof curtain is widely applied to ensure the safety of the foundation pit in areas with multi-aquifer–aquitard alternative strata. The buried depth of the diaphragm wall can influence the environmental effect due to dewatering obviously. This paper investigates the impact of the buried depth of the diaphragm wall on the groundwater drawdown considering the anisotropic permeability of the dewatering aquifer. Numerical simulation is conducted based on an engineering case. The ratio of penetrating depth of diaphragm wall to thickness of dewatering aquifer (R_W) and the ratio of horizontal and vertical hydraulic conductivity of dewatering aquifer (R_C) are varied. The relationship between approximate hydraulic gradient (Δi) and R_W (or R_C) can be fitted by Boltzmann curve (or logarithmic curve). Effective, suggested and control values of R_W (or R_C) are proposed, of which the suggested value is recommended in practical engineering. The effective, suggested and control value of R_W can be calculated by logarithmical equation considering the value of R_C.