The multipath effect is one of the most important characteristics of power line transmission channel. The received signal is the superposition of different transmission path from the transmitter, resulting in frequency selective channel. The characteristics of power line channel impulse response are crucial for communication design and measurement. The conventional research on the power line transmission model only considers point to point channel response. The physical difference between wireless channel and power line channel is ignored. Instead of point to point channel， the whole power line network channel response on multiple ports is studied in this paper. A new channel modeling method under multi-port network environment is proposed. The theoretical channel response relationship among different ports is analyzed. The subnet decomposition and iterative channel response combination methods are established. The theoretical results prove that our proposed method can effectively solve the channel correlation and modeling complexity problem under complicated power line network, and can provide theoretically analysis on the sub-channel correlation and channel capacity of multiple input multiple output (MIMO) PLC system.