Abstract: The gain spectrum of modulation instability is studied in the condition of decreasing dispersion fiber with different group dispersion profiles.The gain spectral width is found to vary as the type of the group dispersion profile changes.The spectral width is discovered simply relevant to transmitted distance for the type of exponential,and the curve of spectral width versus transmitted distance has a vale for the type of linear,Guassian and logarithm,but a firstly broadening and then a gradual decrease is gotten for the type of hyperbolic.Besides the mechanism of pulse compression is interpreted by means of modulation instability.