By changing the modulation frequency of the microwave electro-optic modulator, we achieve the optical frequency scanning. Using the variation of the interference pattern of the backward Rayleigh scattered light generated by the optical frequency shift, we can obtain the information of the sensing temperature change. Based on the cross-correlation properties of the two scanning frequency curves before and after the temperature change and their correspondence with the temperature variation, we propose an optical frequency scanning extension scheme. Theoretical analysis and experimental results show that the proposed scheme can effectively improve the temperature detection range of the system. In the experiment, the signal-to-noise ratio is improved by 2.13 dB, and the minimum detectable temperature change of the system is about 0.029 ℃.