A new method for evaluating the severity of fault lines is presented to overcome the shortcomings of existing methods for exposed area analysis. Based on fault positions method, average voltage magnitude index and the numbers of buses which suffer voltage sag with different magnitudes are used to describe the severity of fault positions. The weights of indexs are determined by using entropy method and the comprehensive severity coefficients of fault positions are then calculated. Average the comprehensive severity coefficient of fault positions which are in a same line to get the comprehensive severity coefficient of each line, and after that the fault line can be sorted considering the effect of voltage sag at multiple buses. Simulation example shows that the proposed method can quantitatively evaluate the severity of fault positions and lines , and can provide some references for adopting sag mitigation measure.