Supercontinuum generated in dispersion-flattened/decreasing fiber is simulated and analyzed in detail. It is found that the peak power of pump pulse is very important to supercontinuum generation from the fiber and there is a threshold power. With the increasing of the peak power of pump pulse, the spectral width becomes wider and the flatness of supercontinuum becomes better. But if the peak power of pump pulse is increased further, the flatness becomes bad. In the end, a optimal peak power for supercontinuum generation is found. It is also found that the fourth and fifth order group-velocity dispersions are crucial to flatly wideband supercontinuum generation from the fiber, dispersion of the third and the other higher than fifth order can be ignored. It is further found that the higher-order nonlinear effects such as stimulated Raman scattering and self-steepening are negligible in the case.