Abstract: Urban flood disasters have occurred frequently in recent years in China, causing a serious threat to urban management and the safety of people and property. The discharge capacity of street inlet is very important to the drainage efficiency in flood-prone areas. Laboratory experiments on the discharge capacity of street inlet were conducted under various flow conditions based on a two-layer platform, covering the upper street layer, the triangular weir tank underneath, and a combination of grate inlet, catch basin and side tube between those two layers. The results show that:the grate inlet was operated like a weir when the catch basin was not full; while the combination was operated as a nozzle after it was full. Based on the experimental data, the discharge coefficients were calibrated for the types of weir and nozzle, respectively. Furthermore, an empirical formula for the discharge capacity of inlet was innovatively derived using the method of dimensional analysis, and the ratio of the composite velocity through inlet to the incoming velocity in front of grate inlet was expressed as a power function of the incoming Froude number. Finally, the proposed formula was validated in detail against existing experimental data for the real storm drain inlet, and it was found that under a higher incoming depth, the discharge capacity of inlet calculated by the proposed formula would be lower than the corresponding reference value adopted widely in practice. This study can provide quantitative basis for refined urban flood management.