In this article,we use Lorentz approximation to investigate the gain properties of multi pumping broadband Raman amplifier,and present some schemes for flattening the gain.All the results show that the flatness of the gain spectrum is closely related to the pumping frequency grid.Researching the gain properties of Raman amplifiers of such ITU T fibers as G.652,G.653,G.655 and large effective area non zero dispersion flattening fiber(LEA NZ DFF),we got that the Raman gain is associated with the fiber type and its transmission characteristics,especially decreasing with the effective core area increasing.