In the paper, we study a single-longitudinal-mode (SLM) erbium-doped fiber (EDF) ring laser with a switchable single-and dual-wavelength output in C-band scale simultaneously. When a 1×4 wavelength-division-multiplexing (WDM) multiplexer is placed inside the ring cavity, four tunable single wavelengths and five sets of dual-wavelength with several mode-spacing can be accomplished, respectively. To achieve the selective and stable output characteristic, the suitable adjustment of cavity and birefringent losses are operated. Additionally, the laser characteristics of output wavelength, detected power, and optical signal to noise ratio (OSNR) have also been discussed.