Aiming at improving signal-to-noise ratio (SNR) and system performance for a Brillouin optical time-domain reflectometer (BOTDR) based on self-heterodyne detection of Rayleigh and Brillouin scattering lights, an approach assisted by modulating probe light and shift-averaging laser frequency, is proposed and demonstrated experimentally. The principles of modulating multifrequency probe light and shift-averaging laser frequency are analyzed theoretically, which are able to increase the signal intensity and reduce the coherent Rayleigh noise, respectively. The experimental results show that the amplitude fluctuations of Brillouin signals and spectrum parameters are decreased effectively, and SNR is enhanced significantly using three-frequency probe light with 22-time shift-averaging compared to that using single-frequency probe light without shift-averaging, and the highest accuracy of temperature measurement obtained on the 50-m-heated fiber near the far end is 0.36°C, which indicates the proposed approach is effective to enhance performance for a self-heterodyne detection BOTDR.