It is important to accurately evaluate the severity of multistage voltage sag for the stable operation of sensitive devices and the governance of voltage sag. This paper presents a method for evaluating the severity of multistage voltage sag based on an improved analytic hierarchy process. First, the multistage voltage sag waveform of the reclosing process under transient and permanent faults is analyzed. Then, the voltage sag evaluation by the weight function method is used to calculate the severity of each stage sag. Based on the time characteristic of process immunity, the multistage voltage sag PIT characteristic curve of a sensitive device is described. Taking advantage of the correlation and severity index value of each stage sag, the weight of each stage sag is determined. The multistage voltage sag severity can be evaluated quantitatively. Finally, the modified IEEE-30 system is used for simulation verification. The results show that the proposed method is reasonable and effective. It reduces the subjectivity of the evaluation results and provides a basis for the evaluation and governance of multistage voltage sag of a sensitive device.