A single longitudinal mode(SLM) erbium-doped fiber laser (EDFL) with multiple ring cavities (MRCs) is proposed and experimentally demonstrated. The EDFL is structured by insertion of three different short passive subring cavities, which serve as mode filters, into the main cavity, and it is combined with a fiber Bragg grating (FBG), the MRCs resonator can make SLM lasing. The resonant theory of MRCs for SLM operation is discussed. At the same time, in order to stabilize, the frequency of output laser, external light injection techniques are employed to effectively suppress the mode jump and beat noise. Output power of 3.6 dBm and signal-noise-ratio (SNR) larger than 35 dB at 1550.225 nm, and linewidth less than 500 Hz are achieved for this laser.