The invention relates to a modulation system and method based on a stimulated brillouin scattering gain spectrum. The modulation system includes a laser, an amplifier, a fiber coupler, a first light polarization controller, an electro-optic modulator, a second light polarization controller, a first optical fiber circulator, a prague grating, a single-mode fiber, a third optical fiber polarization controller, a second optical fiber circulator, a fourth light polarization controller, a polarization analyzer and a polarization state analyzer. The modulation system and method based on the stimulated brillouin scattering gain spectrum adopts the divergence effect of the brillouin polarization spectrum, by changing the polarization direction of the polarization analyzer to select a peak-value wavelength, the spectrum modulation functions of stimulated brillouin scattering gains spectrum compression, peak gains frequency selection, double-peak separation gains spectra and the like are achieved.