In order to realize high-speed and reliable communication in the distribution network， the channel characteristics are studied and three-phase transmission lines are set up in the test field for measuring the signal transmission line in different conditions employing the multi-conductor transmission line theory. The MATLAB simulation is performed to verify the correctness of the results. As the signal transmission law is closely related to the primary parameters of the transmission line， an analysis is made of the parameter matrix calculation of the parameters according to the type of line， material quality and configuration at different frequencies. The results show that it is likely to realize high-speed and reliable signal transmission in rather low attenuation band employing rational modulation.