Single-longitudinal-mode (SLM) fiber lasers are attractive for optical communications and optical sensors. The fiber laser usually operates in multimode and beating noises are a serious problem due to beat between laser mode and amplified spontaneous emission mode. A novel Er-doped single-mode fiber ring laser was demonstrated. In this laser, an unpumped Er-doped fiber was served as a saturable absorber to ensure the SLM operation, and a fiber ring resonator filter was used to suppress the beating noise. With a fiber Bragg grating (FBG) as a selective wavelength component, single mode output without beating noise is obtained experimentally. Measured linewidth is narrower than 5 kHz by using a homodyne setup, which is limited by the radio-frequency (RF) analyzer. The functions of saturable absorber and fiber ring resonator are proved.