The frequency-selecting characteristics of a triple-ring passive resonator are analyzed by Jones matrix， and an erbium-doped fiber laser based on the triple-ring passive resonator is designed according to the frequency-selecting theory of the passive resonators. The experimental results show that the triple-ring passive resonator has a good frequency-selection characteristic. When the pump power is 159 mW， the stable single-longitudinal-mode lasing signal is obtained. The maximum fluctuation of output power is 0.04 dB and the maximum variation of output wavelength is 0.016 nm. The coupling ratios of the optical couplers decrease， the laser will be in multi-longitudinal mode oscillation.