The supercontinuum (SC) spectra generated are compared in dispersion decreasing fibers (DDF) with different dispersion profile. Results show that DDF with concave dispersion profile is more beneficial to generate flat and broad SC spectra, when the dispersion profile of the fiber is symmetrical to central wavelength and decreases from anomalous dispersion region to normal with propagation distance. Optical pulses propagation in fibers has been studied by utilizing total field formation in the frequency field. It is found that optical pulse can be compressed more extensively in anomalous dispersion region, furthermore, optical spectra can be further broadened because there exits a zero-dispersion wavelength in normal dispersion region, leading to the generation of a flatter and broader SC spectra. The spectral bandwidth can reach 298 nm when relative spectral intensity is -27 dB, which is 97 nm broader than that in a convex dispersion profile fiber under the same situation. It is proved by numerical calculation that DDF with concave dispersion profile can generate broader spectrum than that in DDF with convex dispersion profile under the same conditions.