The voltage sag is one of the important problems of power quality. This paper fixates on the question how to generally evaluate the severity of voltage sag rather than to study a single part unilaterally. Therefore, this paper proposes an improved multi-criteria decision-making method. Accordingly, judgment matrices are first built for the problem of multi-criteria decision-making (MCDM) by inquiring experienced experts. Second, the fuzzy preference programming (FPP) extended fuzzy analytic hierarchy process (FAHP) method is presented to calculate the weights of evaluation model containing 3 factors, i.e., characteristic factor, calculation factor and harmonic factor, as well as 6 indicators. Finally, this paper follows the healthy index theory to obtain the final intuitionistic evaluation result by weights and centesimal system. In the validation example of voltage sag, the result of severity evaluation obtained by this method is almost consistent with the actual experience judgement.