The power dependence of phase shift spectrum is very important for the optimization design of vector Brillouin optical time domain analysis (VBOTDA) system. The phase shift spectrum of stimulated Brillouin scattering (SBS) gain was analyzed by mathematical modeling; the phase shift spectrum of SBS gain in 400 m long standard single-mode fiber was measured in a pump wave power range of 5-90 mW and a Stokes wave power range of 5 ?滋W-9 mW by a heterodyne pump-Stokes system; the mechanism of Stokes wave power dependence of SBS gain phase shift was analyzed. The results indicated that the phase shift range of SBS gain had a good linear relationship with pump wave power for a fixed Stokes wave power; the pump power sensitivity of phase shift range of SBS gain decreased from 1.448 (°)/mW to 1.156 (°)/mW with the increase of Stokes power from 5 ?滋W to 8 mW due to pump depletion. According to the theoretical and experimental results, the optimization design of VBOTDA system based on SBS gain was discussed, which provides a basis for the development of SBS gain based VBOTDA system in the field of long distance and high precision sensing.