Optimization of frequency points is needed for the application of Orthogonal Frequency Division Multiplexing (OFDM) technology in power line carrier communication. Based on the analysis of the influence of channel input impedance on voltage transmission characteristics, a frequency selection algorithm based on on-line measurement of channel input reactance is proposed. First, the communication node network is completed under the default frequency, then partial frequency points are selected by online monitoring the channel input reactance, testing the uplink and downlink signal strength at these frequency points. Sub carrier channel frequency points are optimized after simple calculation. The simulation of the actual medium voltage distribution network in Baoding is taken as an example. The simulation results show that the proposed algorithm can filter out a large number of unavailable frequency points under the premise of guaranteeing the communication quality. Thus the frequency optimization speed is improved. This work is supported by the Fundamental Research Funds for the Central Universities (No. 2016MS90).