One challenge with broadband power line communication is the usage of the unshielded transport channels, which still had many interference factors when used to transmit high-speed data, such as noise, attenuation, reflection, radiation and time-varying impedance. In this paper, a two-wire power line is regarded as a long-line antenna. Based on the transmission line theory, reflection theory and radiation loss of long-line antenna, a theoretical model of two-wire power line communication transfer function has been established. The channel transmission characteristics of a power line network can be obtained with its basic elements, the geometric size, the characteristics of the conductor material and the surrounding medium, the structure of power line network, including the power line length, the number of branches and branch terminal load. In the frequency band of 1–200 MHz, the simulated transfer functions of the proposed model are in accordance with the measured results, which proved that the model could accurately predict the power line channel characteristics, and provides theoretical pre-selection guidance of frequency band, power setting and dynamic range for high-speed broadband power line communication.