Generation of supercontinuum (SC) spectrum in the dispersion-flattened decreasing fiber has been studied by utilizing total field formation in the frequency. The initial dispersion and the fiber dispersion-decreasing rate have significant effects on the SC generation. SC bandwidth changes slowly when these two parameters are within certain threshold values, while it changes drastically when they exceed such values. The results also show that convex dispersion fiber is more favorable than the linear dispersion fiber in the generation of wide SC spectrum, while concave dispersion fiber is not suitable for the generation of wide SC spectrum. A 330 nm wide SC spectrum can be generated from a DFDF fiber with optimal dispersion characteristics.