Abstract: In order to shorten the convergence time cost in the power flow calculation of large-scale transmission and distribution network, a method for calculating the global power flow of the transmission and distribution network based on the equivalent fitting of the distribution network power flow is presented in this paper. Firstly, the principle of equivalent fitting of distribution network is analyzed in detail, and the fitting function of distribution network power flow is established. Then, the power flow calculation model of the transmission and distribution network with master-slave splitting and master-slave merging is built. The master-slave iterations are combined with the fitting iteration of the distribution network, the fitting function is solved in the iterative process, and the parameters can be corrected and the fitting error is reduced. In this method, the fitting result of the distribution network and the exact result of the master-slave iterations are taken as the coordination information for boundary points. Thus the master-slave iteration does not needed in each power flow calculation, which reduces the amount of the power flow, accelerates the convergence speed of the power flow and has high accuracy. The simulation results show that the proposed method is feasible and effective, which is suitable for the current global power flow calculation of large scale distribution network.