To improve the accuracy of Brillouin optical time domain reflectometry （BOTDR） in monitoring the state of transformer winding composite optical fibers, a BOTDR sweep range selection strategy is proposed. This strategy generates Brillouin spectra with different signal-to-noise ratios, sweep frequency ranges, and sweep frequency asymmetry coefficients through simulation, and uses the pseudo Voigt model spectral fitting algorithm to calculate the Brillouin frequency shift. Meanwhile, the influence of three factors, namely signal-to-noise ratio, sweep frequency range, and sweep frequency asymmetry coefficient, on the accuracy of Brillouin frequency shift extraction is studied. The experimental results show that when the scanning range is symmetrical around the Brillouin frequency shift, the error reaches its minimum. In the case of a fixed number of sweep frequency points, as the sweep frequency range increases, the frequency shift error first decreases and then increases. When the sweep frequency range is one linewidth larger than the amplitude of the Brillouin frequency shift change, the error reaches its minimum value.