To address the problem of random polarization deflection in fibers, we propose an optimization method for instantaneous frequency measurements based on stimulated Brillouin scattering. The feature difference is analyzed by comparing how a single-mode fiber and a polarization-maintaining fiber affect the realization of instantaneous frequency measurements based on stimulated Brillouin scattering. The experimental results show that random polarization deflection is well suppressed in the polarization-maintaining fiber compared with that in the single-mode fiber. Further, the frequency resolution of the measurement system with the polarization-maintaining fiber is improved from 225 MHz to 90 MHz.