We present a method of achieving a flat-top Brillouin gain spectrum (BGS) with a controllable bandwidth based on multiple-lines pump modulation in a CS2/CCl4 mixture liquid-core optical fiber. The influences of the pump spectrum separation, the intensity of each spectrum line, and the mixing ratio of core liquid media on the BGS are theoretically investigated, and the conditions of producing the flat-top BGS with an adjustable bandwidth are obtained. Results show that 2–9 spectral lines are generated based on single-frequency and multifrequency modulation using an intensity or phase modulator. The flat-top BGS with bandwidths of 50 MHz–2 GHz are obtained by controlling spectrum separation, the intensity ratio of each spectral line and CS2 volume fraction. This method has advantages of convenient operation and wide bandwidth range. It can be applied in Brillouin amplification with high gain and low distortion, and hence satisfies the requirements of weak signal detection and slow light systems.