Converter is an important interface for grid connection of renewable energy, while the interaction between converter and network causes a series of oscillation problems. Because of the confidentiality of the parameters and structure of power electronic devices, the impedance method, as a measurement-based analysis method, is widely used. However, there are some shortcomings in the analysis after obtaining the component impedance matrix, which may bring hidden dangers to the safe operation of power system. In this article, a method of system stability analysis based on impedance model and Gershgorin theorem is proposed. The calculation of eigenvalue loci in the conventional general Nyquist criterion is simplified by estimating the range of eigenvalues using Gershgorin discs, and the relationship between the system stability and the matrix elements is clearly represented. Further, the method is improved to reduce the conservativeness caused by the large radius of Gershgorin discs. Strictly sufficient conditions of system stability and instability, which utilize the matrix elements in an explicitly way, are deduced. The definition of system stability margin is then given. Test results verify the validity and the usability of the proposed method.