The co-operation of stimulated Rayleigh scattering (SRS) in fiber interference loop and stimulated Brillouin scattering (SBS) in fiber can change Q value in resonant cavity, based on which an all-fiber Er-doped Q-switched fiber laser with interference loop is designed. In the experiments, the attributes of output laser exploiting interference loops with different coupling ratios and lengths were investigated. A train of pulse laser with pulse duration of 7.2 ns, repetitive frequency of 212.4 kHz, average output power of 5.4 mW at pump power of 37 mW was obtained which had a good waveform and a peak power fluctuation of ～30%. The results show that the interference loop with very high coupling ratio has no contribution to the generation of pulse laser and the coupling ratio of ～10% is suitable for it. Even if the coupling ratio of ～10% is chosen, the length of the loop has influence on the attribute of the output laser pulse and good output can be obtained at the length around 2 m.