We analyse the stimulated Brillouin scattering （SBS） slow light using double broadband pumps in a single mode fiber. A new SBS gain spectrum tailoring method is proposed using two Gaussian-shaped broadband pumps with different powers and spectral widths, and the central frequency interval of them is carefully set at two Brillouin frequency shifts to make sure that the gain of one pump has the same center with the loss of the other. The top of the gain spectrum of the strong pump beam can be counteracted by the loss spectrum of the weak pump beam to construct a flat top gain spectrum. We give the optimized conditions of the laser powers and spectral widths between two broadband pumps to obtain a flat gain. Distortions of signal pulse under this flat top gain are studied and compared with that obtained using a single broadband pump. We find that the flat top gain profile can effectively reduce the pulse distortions.