In the Brillouin optical time domain reflectometer (BOTDR) fiber-optic sensing system, the scattering signal from Brillouin back scattering is very weak, especially the Brillouin scattering signal at the far end of the fiber, the signal-to-noise ratio is very poor and is very difficult to analyze. A method of improving Brillouin signal quality based on cross-correlation theory was presented. Through cross-correlation with ideal Brillouin scattering spectrum, the difficulty of strain analysis for very weak Brillouin scattering signal at the far end of optical fiber was reduced. It was verified by experiments that this method can effectively improve the signal quality of strain distribution data and the strain sensing accuracy of BOTDR when the signal-to-noise ratio is poor.