Different from the conventional model of dispersion-flattened decreasing fiber, a novel dispersion model of dispersion-flattened decreasing fiber is proposed and supercontinuum generation in is simulated and analyzed by utilizing the total field formation in the frequency domain. It is found that the supercontinuum generated in the latter is by far wider than that in the former and the flat bandwidth can be greater than 1000nm. It is further found that both the fiber parameters, such as peak dispersion, differential constant of the dispersion, and decreasing coefficient of the dispersion, and the pump pulse parameters, including pulse width, soliton order and frequency chirp, are crucial to the flat ultra-wideband supercontinuum generation in the fiber.