To study the hydraulic flow state of a lateral outlet box culvert of a rainwater pumping station in Shanghai, a physical model test method was used to analyze the hydraulic flow state characteristics inside the culvert. Aiming at the founded problems, a combined rectification measure consisting of short diversion piers, bottom sills and composite beams was put forward. The test results show that the water flow inside the outlet box culvert is poorly diffused, resulting in uneven flow distribution at the outlet of the box culvert, centered main flow, and weak outflow on both sides of the outlet. After the combined rectification measures, under the comprehensive rectification action of the short diversion pier, the bottom sill and the composite beam, the flow distribution of each hole of the outlet box culvert is significantly homogenized, which effectively improves the outflow uniformity, and solves the existed problems of the flow state.