As the Thevenin equivalent impedance parameter which is deviated form stability-type simulation data can hardly reflect the electromagnetic characteristics of the electromechanical-side system after fault, an improved calculating method of Thevenin equivalent impedance of electromechanical-side system for electromechanical-electromagentic hybrid transient simulation based on electromagnetic transient simulation model is proposed. The Thevenin equivalent impedance parameter is achieved through setting two three phase line to ground faults on the interface bus in the full-electromagnetic transient simulation model and solving the primary time constant equation group in which the primary time constant is based on a calculating method which is not affected by every harmonic component. Compared with the equivalent impedance calculated by the traditional method which calculates the impedance based on the stability-type simulation data and through unit current injection method, the equivalent impedance calculated with the method proposed in this paper can reflect the electromagnetic characteristics more accurately, thus improving the simulation accuracy of electromechanical-electromagnetic hybrid transient simulation during and after fault.