There are great difference between the calculated dewatering conditions and the actual dewatering conditions, due to the change in the seepage path and seepage area of groundwater of foundation pits. In the paper, the characteristics of the distribution of the flow net of the basic seepage field of foundation pits with waterproofing retaining structures are studied, optimized calculation formula for water inflow of foundation pits is proposed and verified in engineering cases. The study results show that: 1) The distribution map of the flow net of the seepage field around foundation pits can be regarded as two flow net maps cut along the central axis of the retaining structure, and the two flow nets can be used to calculate the water inflow independently. 2) The water inflow of the seepage field inside the foundation pit can be calculated by Darcy law, while that outside the foundation pit can be calculated by the water inflow formula for partially penetrated well. 3) The maximum water level drawdown outside the foundation pit can be obtained according to the functional relationship among foundation pit drawdown, water head, dewatering influence scope and water inflow inside and outside the foundation pit. 4) The optimized formula has been used in the dewatering calculation of actual projects, which demonstrates that the optimized formula can be used in the dewatering calculation of foundation pits with retaining structures. 