A stable single-longitudinal-mode dual-wavelength (SLM-DW) multiple ring fiber laser with a tunable wavelength spacing was proposed and demonstrated. In the multiple ring cavities, the mode selection is made by the tunable digital micro-mirror device (DMD) filter and two different length passive sub-ring cavities. The DMD filter can select and couple any two wavelengths from the gain spectrum of the erbium-doped fiber (EDF). The SLM lasing is guaranteed by the two sub-ring cavities which serve as a mode filter. The stable and uniform dual-wavelength oscillation is obtained by the highly-nonlinear photonic crystal fiber (HN-PCF), which can generate the four-wave-mixing (FWM) effective. By loading different gratings on the DMD filter without any mechanically shift, the tunable wavelength spacing from appropriately 0.165 nm to 1.08 nm within a tuning accuracy of 0.055 nm is achieved at room temperature.