A wide-tunable single-longitudinal-mode fiber laser based on ring cavity structure was presented. The unidirectional propagation of light controlled by the circulator in the ring cavity can eliminate the spatial hole burning effect effectively. For realizing single-longitudinal-mode operation in fiber laser, the compound cavities designed based on Vernier effect were adopted. For suppressing the mode hopping, the saturable absorber effect of an unpumped doped fiber was considered. Meanwhile, the fiber Bragg grating filter with a widely tuning range was used to select oscillation wavelength. With these configurations, a stable single-longitudinal-mode erbium-doped fiber laser with 10 mW output power, 0.7 kHz linewidth and 20 nm tuning range was achieved.