Abstract: Revealing the stability mechanism of the power system forms the foundation for resolving stability issues. However, the reliability of the obtained stability mechanism lacks analytical methods. To solve this problem, the connotation of stability mechanism of power system is analyzed and then the confidence of stability criterion and its derived mechanism is proposed. Next, focusing on the dynamic characteristics of power system in the equilibrium neighborhood, the confidence problem of stability criterion/mechanism is transformed into an evaluation problem of principal contradiction. The identification method of principal contradiction and the confidence evaluation method of stability criterion/mechanism are proposed from the robustness of the stability margin. Further, the reason why many typical frequency-domain criteria of power systems are valid is explained from the perspective of generalized eigenvalues approximating singular values. Finally, the validity of the proposed methods is verified by simulation.