Abstract: Based on the uncertainty of equipment sensitivity to voltage sags, a voltage sag prevention index is proposed for the virtual upper and lower limit curve with single tolerance characteristic. Firstly, the virtual upper and lower limit curves are described according to the uncertainty of the equipment’s tolerance characteristics, and on this basis, a voltage sag severity evaluation model is established based on the virtual upper and lower limit curves to visually present the sag’s impact on the equipment. And then, with consideration of user mitigation expectation threshold, a compensation cost function and an economic benefit based profit function are established, and the voltage sag prevention index is defined. Finally, the effectiveness and rationality of the proposed method is verified through a case study. The proposed method can be used for voltage sag evaluation and governance of actual power systems.