Abstract: The interaction, existing between the plant and other components of power system, seriously affects the characteristics of power system and causes a fatal risk. To fill the gap, a unification stability criterion for the power electronics dominated power system was proposed in this paper. Firstly, the definition and calculation method on unification stability index of dynamic state (USI₁) and unification stability index of static state (USI₂) were expressed, and their theoretical basis and physical meaning were discussed in detail. Secondly, the stability assessment method for power system was proposed on relative gain array (RGA) theory and USI. Then, a three-dimensional index set was constructed to analyze the stability among plant, subsystem and power system. Finally, an example was given to verify the effectiveness of the proposed method for the power electronics dominated power system.