In this paper, the optimum supercontinuum (SC) spectrum generation in a dispersion decreasing fiber is presented. Three normalized parameters for the pump pulse and SC fiber are introduced. It is found that the shape of an SC spectrum is uniquely specified by the input soliton order, the normalized dispersion slope and the normalized effective fiber length. For a pumping condition with a given input soliton order and a given normalized dispersion slope, by optimizing the normalized effective fiber length, the residual spectral peak in the SC spectrum can be suppressed effectively, and a broad SC spectrum with optimum spectral flatness can be obtained.