In response to the challenges in stability analysis posed by the multiple-input multiple-output (MIMO) characteristics in power systems, this paper innovatively introduces input-output stability theory and the small-gain theorem into the calculation of system stability margins. A feedback interconnection framework of "system-disturbance" is established, along with a quantitative method for assessing stability margins. The proposed approach comprises two key contributions: first, the computation of the maximum singular value of the system to establish a robust stability criterion for MIMO systems, which effectively characterizes the coupling characteristics among variables; second, the application of the small-gain theorem to develop an explicitly quantifiable gain margin index, thereby enabling the computation of stability margins that systematically account for system uncertainties. Simulation results demonstrate that the proposed index accurately reflects the system's disturbance rejection capability and the influence of control parameters on stability, even under complex variations in system dynamics. This method provides a unified analytical tool for evaluating the stability characteristics of diverse devices in power systems, offering significant theoretical and practical value.