In this paper, the self-starting mode-locking characteristics of all fiber lasers with nonlinear amplifying fiber loop are analyzed and the theoretical model of calculating the pulse effective energy gain in the lasers is obtained. The resultS show that the selfstarting mode-locking of the lasers are related to the gain and cross-section of the Er3+-doped fiber. The threshold peak powers of the pulses for self-starting mode-locking are calculated. With this model, the experimental phenomena are well explained.