The invention relates to a method and system for evaluating the severity degree of voltage sag. A weight function of a voltage sag performance influence degree is calculated according to a voltage sagtolerance curve; on the basis of the weight function, a voltage sag performance influence degree is calculated, wherein the voltage sag performance influence degree is used for expressing a severitydegree of a voltage sag event; according to actually measured voltage values of all sampling points during a voltage sag process, time parameters of the corresponding sampling points are calculated respectively and the time parameters of all sampling points are introduced into the voltage sag performance influence degree to obtain single voltage sag performance parameters of the corresponding sampling points; and the largest voltage sag performance parameter among the voltage sag performance parameters is set as a single voltage sag severity degree of the voltage sag event.