The invention discloses an electromagnetic transient stability device of a distributed power generation system and a method. The electromagnetic transient stability device is connected into an alternating current bus of the distributed power generation system in parallel, and comprises a power unit, a filter unit, a detecting unit, a control unit and an impedance calculation unit, wherein the output end of the power unit is connected with the input end of the filter unit; the output end of the filter unit is connected into the alternating current bus of the system in parallel; the input end of the detecting unit is connected with the output end of the filter unit; the input end of the impedance calculation unit is connected with the output end of the detecting unit; the output end of the impedance calculation unit is connected with the input end of the control unit; and the output end of the control unit is connected with the control end of the power unit. The detecting unit samples output voltage and current signals of the electromagnetic transient stability device; the impedance calculation unit calculates the impedance of an external system according to the output voltage and current signals; the control unit resets the self impedance of the device according to the impedance, so that resonant vibration is damped; the grid-connected current waveform quality is improved; and the grid-connected stability is improved.