In order to reduce the impact of white noise and random multipath noise in power grid optical communication system, two monitoring schemes based on linear frequency modulation-optical time domain reflectometer(LFM-OTDR) technology are proposed, namely single frequency LFM-OTDR scheme and frequency division multiplexing LFM-OTDR(FDM-LFM-OTDR) scheme. In this scheme, LFM optical pulse signal is used as the detection signal, and the backscattered detection signal is transformed into the fractional Fourier domain by fractional Fourier transform for signal processing and scattered light power calculation, so that the detection signal and backscattered noise can be separated more effectively. Compared with the single pulse OTDR scheme, the experimental results show that the dynamic range of OTDR is improved by 8.3 dB and 11 dB respectively by using single frequency LFM-OTDR scheme and FDM-LFM-OTDR scheme.