The mechanism of a novel structure self-Q-switched erbium-doped fiber ring laser has been investigated. The results show, according to phase difference of counterpropagating light fields in a fiber loop, the optical feedback which is consisted of afiber coupler and LD chip can feedback light fields into the erbium-doped fiber selectively, and the laser can generate optical pulses with increasing amplitude. Optical pulses lead to transient gain saturation of the erbium-doped fiber, which will result in self-Q-switching in the laser. The theoretical analysis is in good agreement with the experiments.