The system decoupling algorithm is a prerequisite for the establishment of the co-simulation. Based on the study of the traditional decoupling algorithm for transmission lines, this paper proposes an improved transmission line interface algorithm for cross-platform joint simulation of wind farms into large-scale AC and DC grid based on RT-lab and Hypersim. And Matlab simulation results show that the improved interface algorithm can effectively improve the accuracy and stability of the co-simulation. This co-simulation combines the advantages of each software to realize accurately simulation results of large-scale new energy accessed to grid，the wind farm model is established in RT-LAB, and the AC / DC network is established in Hypersim.