In order to analyze the influence of related factors of demodulation error in Brillouin optical time-domain reflectometry/Brillouin optical time domain analysis(BOTDR/BOTDA), firstly, the noisy Brillouin spectra with different parameter values is simulated generated based on the Lorentzian profile. Then the Brillouin frequency shift error in the different conditions is calculated by use of the least-squares fitting method based on the Lorentzian profile. The results reveal that when the number of sweeps is fixed, the frequency scanning range is not recommended to be too large or too small, and the recommended frequency sweep range is 1.2ΔvB. When the frequency sweep range is constant, the Brillouin frequency shift error decreases exponentially with increasing number of frequency sweep. The Brillouin frequency shift error decreases exponentially with increasing signal-to-noise ratio. When the ratio of frequency sweep range to linewidth and the number of frequency sweep are constant, the Brillouin frequency shift error increases approximately in direct proportion with the increase of Brillouin linewidth.