In order to confirm relationship between flow rate of foundation pit and suspended curtain depth into stratum and relationship between maximum water level reduction of outside the foundation pit, so then finding the optimum value of suspended curtain depth into stratum, according to Darcy’s law and large-well-method, formula was derived. By comparative analysis of example of derived formula and Davidenkoff's way,the similarities and differences of the results was discovered. Groundwater seepage in foundation pit was simulated by laboratory model experiment and Midas/GTS/NX, verifying derived formula and coming to conclusion, which maximum water level reduction of outside the foundation pit was discussed slightly with increasing of suspended curtain depth into stratum and flow rate of foundation pit was reduced.