The invention relates to the technical field of subsynchronous oscillation modal signal measurement, and discloses a power generator subsynchronous oscillation modal signal measuring device and method. The device comprises a signal generator which is used for outputting an excitation signal; a measuring device which is used for measuring rotating speed change amplitude of a power generator; and a processing device which is connected with the signal generator and the measuring device and determines the attribute of a subsynchronous oscillation modal signal via the attribute of the excitation signal and the rotating speed change amplitude of the power generator generated after the signal excited by the excitation signal affects on the power generator. The attribute of the modal signal causing subsynchronous oscillation of the power generator can be determined via the corresponding relation between the excitation signal obtained after the excitation signal affects on the power generator and the generated shafting oscillation of the power generator so that any special equipment does not need to be additionally arranged in the whole process, operability is high, economic benefit is substantial, and thus the device has important meaning in solving a subsynchronous oscillation problem of a power plant and an electric network.