The coincidence among meteorological parameters in the traditional design day of air-conditioning system design is not considered sufficiently， resulting in the design cooling load of air-conditioning system being far from the actual peak cooling load. In order to select the reasonable coincident design day for air-conditioning system design load， a method based on the meteorological daily ordinal number is proposed to select a coincident design day for air-conditioning system. In this method， the concept of meteorological daily ordinal number is first defined. Then， the dynamic cooling load of room is calculated using historically measured meteorological data and dynamic cooling load calculation mode， and a general daily ordinal number set is selected according to indoor thermal environment risk level. At last， a multi-dimensional and multi-parameter clustering analysis method is proposed to select a representative coincident design day from the general daily ordinal number set. This method is applied to select the coincident design day of Hong Kong and Changsha from their long-term measured meteorological data. A case study shows that the number of coincident design days determined by this method is few， which suits engineering design application. Comparison with the traditional design day shows that the coincident design day determined by this method has better accuracy in calculating design cooling load and can well meet the requirements of engineering design accuracy.