Impedance mismatching is a major obstacle hindering the application of DC power line communication (DC-PLC) due to the unpredictability of access impedance and random loads. Researchers and manufacturers typically estimate the power line impedance level and use a fixed single-winding coupler to carry out signal coupling, which does not achieve accurate impedance matching and leads to large signal attenuation and low reliability. In this paper, a lumped parameter power line communication model for DC-PLC is established in which the optimal receiver winding ratio is derived from equivalent circuits. A modifiable impedance matching coupler was designed to achieve dynamic impedance matching, and a series of simulations were run to analyze the relationship among optimal winding ratio, power line impedance and series loads. The performance of different winding ratio couplers under varied frequency and load impedance was also measured in a laboratory environment to find that adopting the modifiable impedance matching coupler is indeed a useful strategy for achieving adaptive impedance matching with maximum signal power transfer.