An optical time domain analysis system that is based on the interaction between the Brillouin gain and loss is proposed. A continuous wave propagates with reverse pulses at its Stokes and anti-Stokes frequencies in an optical fiber. Further, a narrow linewidth absorption peak is observed at the center frequency of the Brillouin gain spectrum. The linewidth of the absorption peak is approximately 1/5 of that observed in the Brillouin gain spectrum, and the frequency of the absorption peak is related to the Brillouin frequency shift. Therefore, the frequency resolution of a Brillouin sensor can be improved by the narrow linewidth absorption peak; thus, highly accurate temperature or strain measurements can be achieved. The experimental result depicts that the accuracy of the temperature measured by this scheme is more than doubled, compared with the traditional Brillouin gain spectrum based sensor.