A method of using monitored hydrological data to assess the degree of inlet blockage was proposed in order to quantitatively analyze the uncertainty of the drainage system. The correlation between the monitored hydrological data and the blocked area of the inlet was obtained by describing the physical model of the road drainage inlet under flooding. The average clogged area of the inlet under each independent water accumulation event was calculated and defined as the index of inlet’s blockage degree. The moving average method was used to process the calculation results in order to visually analyze the degree of inlet’s blockage. The method was applied to an eastern coastal city, and the inlet cleanup records were used to verify the calculation results. Results show that 83% of inlet blockage can be well identified, of which the accuracy of the points diagnosed as cleared is 75%, the accuracy of the points diagnosed as uncleared is 89%. Errors in the calculation results are mostly caused by incomplete records. Some branch pipes may be gradually cleaned up after the inlet is cleared, which interferes with the calculation results.