A wavelength-switchable single-longitudinal-mode (SLM) dual-wavelength erbium-doped fiber laser (EDFL) incorporating a novel high-finesse ring filter is proposed and demonstrated. The ring filter consists of two optical couplers and a section of pumped erbium-doped fiber (EDF). Thanks to the gain generated by the EDF, the ring filter has spectral response with a high finesse. The incorporation of the ring filter leads to the suppression of undesirable modes in the dual-wavelength EDFL. An experiment is carried out. Two SLM wavelengths are generated. The side mode suppression ratio is greater than 50 dB. The wavelength spacing of the two wavelengths is tunable with a tuning step of approximately 10 GHz. A frequency switchable microwave signal from approximately 10 to approximately 40 GHz is thus generated by beating the two wavelengths at a photodetector (PD). The spectral width of the generated microwave signal is measured to be less than 5 kHz.