The invention relates to a method for designing en interface for an electric system, in particular to a method for designing an interface with harmonic characteristic hybrid simulation function. The method comprises the following steps: 1, building a Thevenin equivalent interface matrix and an interface equivalent voltage source; 2, building a frequency-dependent equivalent model; and 3, building an equivalent interface circuit of a multi-frequency harmonic power supply. The method not only combines the interface model frequency harmonic characteristics of an electromechanical transient subnetwork, but also utilizes the harmonic source characteristics of the electromechanical transient subnetwork, can transmit the harmonic characteristics of the electromechanical transient subnetwork to an electromagnetic transient subnetwork for calculation, and increases the precision and reliability of the simulating calculation.