In order to improve the real-time performance of optical fiber strain measurement based on Brillouin scattering, the double slope-assisted technique, which calculates Brillouin frequency shift based on Brillouin gain at two working points, was used for optical fiber strain measurement. Firstly, the basic principle of double slope-assisted technique was introduced. Secondly, the influence of the left and right working points on the accuracy in Brillouin frequency shift was investigated based on simulated Brillouin spectra, and the optimal working point was determined. Finally, based on the determined optimal working point and Brillouin spectra with different values of SNR (signal-to-noise ratio), the influence of SNR on the accuracy of least-squares fitting method and the double slope-assisted technique was investigated. The results reveal that if the two working points are symmetrical around the mean value of Brillouin frequency shift along the optical fiber, the error in Brillouin frequency shift attains a minimum value. The Brillouin frequency shift error decreases first and then increases with the frequency difference between the two working points. When the accuracy is similar, the measurement time of Brillouin spectrum of the double slope method is only about 1/30 of the least-squares fitting method. The calculation time of Brillouin frequency shift by the double slope-assisted technique is only about 1/480 of that of the least-squares fitting method.