In order to apply Nash Unit Hydrograph to ungauged basins, we studied the parameter regularity of Nash Unit Hydrograph based on the DEM data. The 29 medium and small basins in Wannan mountainous watershed are used as experimental basins to calibrate the parameters of Nash Unit Hydrograph. Geomorphic parameters and surface land cover types obtained by ArcGIS 10.1 are adopted to make multiple regression analysis and path analysis of the parameters, and establish the formula which calculates the parameters of Nash Unit Hydrograph based on the DEM. The result of regression analysis shows that the formula has high statistical significance. The result of path analysis shows that slope is the fundamental explanatory variable, which plays a decisive role in calculating the parameters of Nash Unit Hydrograph. The formula independent on measured data can be generalized and applied to ungauged basins.