Brillouin spectroscopy is an important topic and powerful tool in modern optics, as the acquisitions of acoustic velocities and elastic moduli are one of the keys to investigate and analyze the contents of material science and condensed matter physics. Although stimulated Brillouin spectroscopy based on the pump-probe technique has striking advantages that include higher spectral resolution and signal-to-noise ratio, it is challenging to accomplish high-speed acquisition in the presence of pump background noise. In this paper, we propose a method for signal–noise separation through spiral phase precoding of the Brillouin spectrum signal. We achieve on-demand tailoring spatial distribution of the signal, and hence the signal can be separated from the background noise. Furthermore, this approach has little energy loss due to phase-only modulation, and retains the advantages of high efficiency and high gain in Brillouin interaction. The proof-of-principle demonstration provides a practical way to reshape the spatial structure of Brillouin spectra, and shows the potential in quasi-noise-free nonlinear interactions.