Voltage transmission characteristics of power line communication (PLC) channel are asymmetric in medium voltage distribution networks. It is found that the asymmetry reduces the efficiency of establishing connection when the PLC device is tested at the medium voltage distribution network. In this paper, a PLC channel model is established based on the partial reflection theory. Then the forward and backward voltage transmission characteristics are analyzed in typical topologies of medium voltage distribution networks. The results show that under the condition of homogeneous trunk channel or overheadcableoverhead trunk channel, channel characteristics can be only analyzed in either direction of forward or backward. And for the condition of overheadcable trunk channel, only the channel characteristic of overhead to cable needs to be analyzed. The lateral network among trunk channels has a shunt effect on PLC signals. The channel quality is poorer when the signal is transferred from the terminal with a stronger shunt effect. In addition, the PLC device can select a better communication frequency by detecting the reactance component of the online input impedance.