Based on fiber ring lasers, we designed a single-wavelength and dual-wavelength switchable single-frequency ytterbium-doped fiber laser. A high-finesse filter was composed of a three-port circulator, an unpumped ytterbium-doped fiber and a fiber Bragg grating, which was used to suppress the number of modes in the resonator. By tuning polarization controller, comb spectra and dynamic gratings were formed within Polarization Maintaining Ytterbium Doped Fiber(PM-YDF) and realized the output of a single-frequency fiber laser with narrow linewidth. The output linewidth of the laser was 346 Hz at 1064.37 nm, and the optical signal-to-noise ratio was greater than 50 dB. The instability of wavelength and power was within 0.01 nm and 0.2 dB in 30 min. By adjusting the polarization controller, the single and dual wavelengths could be switched to each other, and the dual wavelengths were located at 1064.156 nm and 1065.236 nm, respectively. This technology provides a new way for dual wavelength output of ultra-narrow linewidth lasers.