The flow pattern of aerosol and sheath air in a cylindrical annulus of a Differential Mobility Analyzer (DMA) is critical to determine the classification performance of the DMA. In the present work, a new technique to check aerosol flow inside the DMA is proposed and the influences of aerosol flow distortion and broadening on the classification performance are investigated by means of the Tandem DMA method. As a result, it is found that (1) the annular flow inside the studied DMA is a fully developed laminar flow with a negligible mixing between aerosol and sheath air when the aerosol flow rate is less than 3 l/min and the sheath air flow rate is 17.5 l/min, (2) a larger width of aerosol entrance slit causes uneven distribution of aerosol flow in the circumferential direction, and (3) both the uneven distribution and broadening of the aerosol flow lead to a broader band of classified particle electrical mobility, as well as a shift of peak electrical mobility to a smaller range. These results suggest that aerosol flow inside a short-type DMA is more sensitive to the flow rates and its structure compared to the conventional TSI type DMA.