With the wide application of the broadband power line communication technology in AMI system, the most urgent need is to set up a digitization integrated test environment for the carrier communication. The existent testing method is mainly derived from the conditional point-to-point power line channel model. Due to the multi-stage relay routing mechanism in carrier communication, the multi-node channel model is proposed for the link-level and system-level performance testing. Based on the two-port network theory, the multi-node broadband power line channel modelling approach is presented. With this method, the system model is simplified through the sub-network centered by branch node and the mechanism of the correlation between different channels in the low voltage distribution network is analyzed under the classical T-shape network. Through comparing real-measured channel response with theoretical simulation result, it verifies that the proposed method can accurately describe multi-node power line channels and channel correlation, which can offer theoretical support for the system-level testing approach and novel high speed broadband power line communication in the future. This work is supported by China Postdoctoral Science Foundation (No. 2015T80961) and Natural Science Foundation of Chongqing (No. cstc2016jcyjA0214).