In view of the confusion of using different Allan variance to analyze the sensitivity of atomic interferometer, this paper systematically gives the detailed derivation of Allan variance, overlapping Allen variance and modified Allan variance in time domain and frequency domain, analyzes their resolution to five kinds of typical noise, and points out that the modified Allan variance is more suitable for evaluating the long?term stability of atomic interferometer. Based on the expression of modified Allan variance in frequency domain, the transfer function between sensitivity and noise power spectrum of atomic interferometer with measurement dead zone is given for the first time. By analyzing its characteristics, two specific ways to improve the sensitivity of atomic interferometer are pointed out. These studies have laid a more solid theoretical foundation for the further development and evaluation of atomic interferometer technology.