A novel method by demodulating the sideband of Brillouin gain spectrum (BGS) is proposed and demonstrated in order to enhance temperature measurement accuracy in a Brillouin optical time domain reflectometry (BOTDR) sensing system in this paper.Firstly,the characteristic of frequency shift of an acoustic optical modulator (AOM) is utilized to generate the sideband of BGS,and the influence of the peak power of the probe optical pulse on the temperature measurement accuracy is also investigated.Moreover,the theoretical analysis shows that benefiting from the reference continuous light from the source laser by the coherent detection,the intensity of the sideband is higher than that of the central peak,which indicates that the higher signal-to-noise ratio (SNR) can be expected by demodulating the sideband of BGS instead of the central peak.Thus the demodulating the sideband of BGS can further improve temperature measurement accuracy in the BOTDR sensing system theoretically.Secondly,the experimental setup of the distributed temperature sensing system based on BOTDR is built.The AOM is selected as the optical pulse modulator to produce high-extinction-ratio probe pulse light,following the frequency upshift of the injection light.The beat signal generated by coherently detecting the backscattering light from the fiber under test (FUT) and the reference light from the source laser is acquired.Furthermore,the central peak and the left sideband of BGS are respectively scanned by using microwave heterodyne frequency shift technique.The time domain waveforms at each frequency point are then obtained and Lorentzian curve fitting is performed at each sampling position,thus Brillouin frequency shift (BFS) along the FUT is plotted and the temperature is demodulated along the FUT based on the linear dependence of the BFS on the temperature in the optical fiber.Finally,the experimental results show that the peak power of the left sideband of Brillouin gain spectrum is about 3.27 dB stronger than that of the central peak.Meanwhile,the linewidth of left sideband of BGS is about 14.7 MHz narrower than that of the central peak at -1 dB point in the same conditions.When the left sideband of BGS is scanned,the SNR of the BOTDR system is improved by 4.35 dB due to the contribution of the reference light by coherently detecting and eliminating the effect of the coherent Rayleigh noise,and then the temperature measurement accuracy of 0.5℃ is achieved over a 10.2 km sensing fiber.