The Interline Power Flow Controller (IPFC) can realize the transfer and distribution of power flow between lines, which can be used to solve a series of problems caused by uneven power flow in the power system, and has great potential and value for application. It is necessary to implement the power flow calculation of large-scale power system with IPFC for evaluating the application value of IPFC. However, at present, there is no IPFC model developed in large-scale power system analysis software used in power grid planning and design in China. To solve the above problem, this paper proposes a joint power flow calculation method of power system with IPFC based on Matlab and PSD-BPA. Firstly, the mathematical expression of the IPFC power injection model is derived, and the calculation framework of joint power flow calculation between Matlab and BPA is proposed. In the proposed framework, Matlab performs the calculation of IPFC, BPA performs the power flow calculation of large power grid, and the data exchange interface completes the interaction and alternate solution of two simulation software. Furthermore, the IPFC power injection model is improved, and an iterative strategy with variable step size based on the PI controller is proposed to improve the convergence of the proposed method. The proposed method is simulated and verified in the Nantong Northwest Power Grid. The results reveal the correctness and effectiveness of the proposed method. This work is supported by Science and Technology Foundation of State Grid Corporation of China (No. SGJSJX 00YJJS1800739).