Abstract: Street inlet is one of the key facilities in urbran drainage systems. Once street inlets are clogged, the surface water cannot be discharged efficiently into the underground pipes, which is one of the main reasons for the occurrence of severe urban flooding. In order to quantitatively investigate the influence mechanism and degree of street inlet clogging on its discharge capacity, experiments were carried out in a specific laboratory flume based on the condition of large incoming water depths, considering different clogging degrees of grate and lateral side tube respectively. A total of 608 groups of steady flow experiments were conducted. Experimental results show that the clogging of both grate and side tube can significantly influence the comprehensive discharge capacity of a street inlet, with the clogging of side tube having a more pronounced effect. The discharge coefficients were determined for weir and nozzle flows using these experimental data. Based on the dimensional analysis and the regression method, the formula of inlet discharge capacity expressed by a power function of clogging degree was proposed, and the coefficients were calibrated by experimental results. These results show that the latter calibrations based on the power function expression are better than that for the weir or nozzle formulae. The relationships between clogging coefficient and clogging degree were proposed respectively for inlet and side tube clogging, based on the non-linear fitting method and mean assumption of the power exponent. The unified formula for discharge capacity of street inlet was finally established, which can not only account for the weir and nozzle flow patterns, but also can reflect the effect of different clogging degrees of grate and side tube, and the proposed formula is applicable to large incoming water depths