In order to solve the problems in mode hopping detection for single-frequency fiber lasers, a method is put forward based on the dynamic phase changes in an unbalanced Michelson fiber-optic interferometer. Steep frequency changes in mode hopping process are transformed to steep phase changes in the working point of the interferometer, and the changes are obtained accurately by tracking simultaneously both the sine and cosine functions of the working point in time domain. Various mode hops in an ultra-narrow linewidth erbium-doped fiber ring laser, including occasional hops and thermally-induced serial hops the moment the laser is switched on, have been detected successfully with this method. The results show that this method has a high testing sensitivity, and makes real-time and apparent criterions for identifying mode hops. Besides, possible frequency drift between two serial mode hops can also be measured by use of this method, which plays an effective role in evaluating the lasing mode stabilities of single-frequency fiber lasers.