Abstract: In hybrid electrometrical and electromagnetic transient simulation, the equivalent modeling of the other side system should be concerned when the calculation is carried in one side. To simulate the electromechanical side system accurately, an interface modeling method for hybrid electromechanical and electromagnetic transient simulation is proposed based on RTDS/CBuilder. The modeling principle of interface model is introduced in detail as well as realization method and calculation scheme. The simulation results show that this method can ensure the simulation accuracy when an imbalanced fault occurs in the electromagnetic side, which has good scalability.