In order to study the influence of road slope(1%-2% in cross slope and 0. 3%-5. 0% in longitudinal slope)and runoff of 0-70 m³/h on the inlet efficiency, tests were carried out on a 1∶1 platform. Experimental results show that the inlet efficiency decreases with the increasing of flow rate as well as the longitudinal slope. However, when the cross slope increases 0. 5%, the efficiency increases about 10%. In addition, experimental data were fitted with the orifice equation. Comparison was conducted between the fitting formula and previous empirical formulas, and the results show that the proposed formula gives better results on the standard flat-grate street inlet. Based on the fitting formula, a new method for the street inlet design is proposed for the future urban road construction and formulas for determining the inlet spacing are deduced, which were adopted for the optimal design of inlet spacing.