To solve the problem that the obtained stimulated Brillouin scattering(SBS) threshold from the measurement of backscattered power and forward transmitted power was higher than its real value, a new method employing Brillouin scattering spectrum width for fiber SBS threshold determination was proposed. The principle of local optical heterodyne detection and the relationship of the Brillouin scattering spectrum width with the fiber input power were theoretically analyzed. Experimental setup for Brillouin scattering spectrum measurement based on local optical heterodyne detection was designed, and the SBS thresholds of two standard single-mode fibers with different length were measured at room temperature. The results show that the Brillouin scattering spectrum widths corresponding to the SBS thresholds of 48.8 km and 9.5 km long fibers obtained from the power information are all approximate to a constant value of 10 MHz, and the serious pump depletion effect has occurred. The SBS thresholds are 1.12 mW and 3.8 mW for 48.8 km and 9.5 km long fibers determined by using the characteristics of the linear decline of Brillouin scattering spectrum widths at low fiber input power and the power independence of spectrum widths at high input power presenting in the Brillouin scattering spectra. The corresponding Brillouin spectrum widths are 24.86 MHz and 23 MHz, respectively, which are approximately equal to the natural Brillouin linewidth. The results of this study can provide a theoretical basis for the determination of the maximum fiber input power in Brillouin optical time domain reflectometer system.