Abstract: In this paper, an evaluation method of voltage sag severity is proposed based on entropy coefficient method. First of all, based on IEC61000-2-8 statistical tables, evaluation index and hierarchy classification are determined. Then, the influence degree of duration and amplitude is calculated based on the analytic hierarchy process(AHP) and weight function method, and it is further assumed that the duration and amplitude are independent of each other, by which influence degree of voltage sag is defined in the plural form, and influence degree is quantified by standardized modulus value. In the end, the comprehensive influence degree of voltage sags is obtained by entropy method through weighting summing influence degree of voltage sag by combining analytic hierarchy process and weight function methods. The analyzing results of monitoring data of the grid voltage sag for a certain city obtained by using proposed method show that the calculation results obtained by combing analytic hierarchy process and weight functions method through entropy coefficient method are more reasonable than that obtained by single method, and the proposed comprehensive effect index of voltage sag can more fully describe the voltage sag severity of the typical monitoring stations in power grid.