The open sump is a typical inlet passage of middle and small pumping station. The design parameters of sump affect hydraulic performance of pump. The incompressible N—S equations are solved by the finite volume method. Base on the RNG k—ε model, the SIMPLEC algorithm is applied for the solution of the discretization governing equation. The results illustrate that, excessively small backwall-clearance (T) may lead to unidirectional flow above bellmoth and excessively large T may cause backflows near back wall. It can be observed that curvature of flow near bellmouth appears large while the bottom floor clearance (hc) is too small, but excessively large hc may cause larger head loss and non-uniform axial velocity distribution of blade inlet. The results show that the hydraulic performance of rectangle sump, trapezia sump and semicircle sump are similar, and the best one is ω-shape sump because the distributing flowfield of suction pipe is uniform. The numerical simulation results show that the recommenced parameters brought out and the best plane back wall shape and suitable dimensions of sump presented could be applied in projects.