Abstract: To improve the precision of power flow analysis in beam-plate system, the connection load between beam and plate is introduced in the process of decoupling of power flow model to analyze character of power flow in coupled system. The results are compared with those from Finite Element Method, which prove that the model considering connection load is more accurate. It also demonstrates that the connection load have to be included in coupling model. Researches on effects of structure parameters on difference between results from model considering and neglecting load are carried out to obtain an error principle of power flow analysis in coupling system. The simulation results show that the discrepancy between results from models with and without considering load becomes larger with decreasing of the boundary stiffness and increasing of the length ratio between beam and plate, which means that the need of introducing connection load grows up in this situation. When the thickness of plate is close to that of beam, the outcomes form models with and without consideration of load model are similar. For convenience in calculation, it is not necessary to take the connection load into account.