Power Line Communication (PLC) is one of the important choices for smart distribution network communication technology. The Medium Voltage (MV) distribution network has complex structure, numerous branches, and cable-overhead mixed lines, which make the transmission characteristics of the carrier signal difficult to predict. Based on the structural characteristics of an MV distribution network, this paper proposes a channel modeling method for carrier signals in mixed distribution networks. First, the transmission line pipeline of the carrier signal is defined based on the actual topology and line type of the MV distribution network. Then, the propagation matrix of each transmission line pipeline and the scattering matrix of each node can be derived based on multi-conductor transmission line theory and traveling wave theory, and the voltage equation of the whole network can be written according to the network topology relationship. Finally, the voltage response of all nodes in the whole network under the excitation of the source node can be obtained by the voltage equation. The modeling method is not restricted by the network topology nor line type, and has strong adaptability and portability. The results of calculation and circuit simulation show the correctness and effectiveness of the method. This work is supported by the National Natural Science Foundation of China (No. 51807063) and the Fundamental Research Funds for the Central Universities (No. 2019MS081).