Several serious accidents caused by subsynchronous oscillation (SSO) have occurred. In order to study the subsynchronous oscillation, the impedance matrix and the characteristic equation are analyzed. By means of equivalent circuit, the physical meanings of the characteristic equation and impedance are analyzed from the view of impedance. Then, the eigenvalues of the oscillation mode of the system are estimated according to the frequency impedance curve. A quantitative analysis method based on the impedance method is proposed, and the deficiency of existing stability analysis methods based on impedance model is resolved. Finally, the practicability and effectiveness of the impedance stability analysis method are verified through simulation case.