A scheme for fast temperature measurement based on distributed Brillouin optical fiber sensor is proposed. The Brillouin scattering signal is detected by the high speed pulse RF detector based on the principle of heterodyne coherent detection, with which the measuring time can be decreased. Moreover, the noise of the Brillouin scattering signal is decreased by combining cumulative average technology with wavelet transform technology. Thus the de-noised signal is demodulated for temperature measurement within large distance. The experimental results make it clear that a temperature measurement error of less than 3 ℃ and a measuring time of less than 3 s can be achieved at a distance of 24.8 km.