A high power, single-frequency, narrow linewidth fiber laser based on an active double-ring cavity under optical injection is proposed. The laser produces power of 36 dBm, signal-to-noise ratio (SNR) of 40 dB and linewidith of 7.1 kHz. The wavelength and power are stable within several hours of observing time. The allfiber double-ring cavity consists of two sub-ring cavities: the main-ring cavity is gain-controlled through adjustment of the pump power and the intracavity feedback ratio, however, the sub-ring cavity is injected by a master laser whose power and wavelength are tunable. To achieve single-frequency lasing, the intracavity gain and oscillation mode′s frequency of double-ring cavity should be matched with the optical power and frequency of the master laser, respectively. This frequency locking condition is achieved by adjusting the pump power and variable optical coupler. The single-longitudinal-mode selection regime is analyzed by a semi-classical multimode oscillation theory. Simulation results are in good accordance with the experimental results. This fiber laser can be useful in current coherent communication systems and distributed optical fiber sensors.