The method of generating equal-amplitude spectral lines by multi-frequency phase modulation is used in stimulated Brillouin scattering (SBS) suppression. The spectra of three, five, seven, and eleven equal-amplitude spectral lines are obtained in experiment with flatnesses less than 0.3 dB. Theoretical research on SBS suppression shows that the threshold power after modulation is in reverse proportion to the maximum square of amplitude moduli of fundamental frequency and the nth harmonic wave. The threshold powers of three, five, seven, and eleven equal-amplitude spectral lines are improved by 5.21, 8.36, 9.39, and 10.76 dB, respectively.