A semiconductor optical amplifier (SOA) multi-wavelength fiber ring laser with a Mach-Zehnder (M-Z) fiber interferometer was reported and stable multi-wavelength continuous-wave lasing oscillations with channel spacing of 100 GHz are achieved with the 3 dB spectral bandwidth of 19.5 nm and signal-to-noise ratio higher than 30 dB. It is observed that the output power variation of the 22 wavelengths in 17.9 nm is 1.2 dB and total output power is 5.1 dBm. The wide spectrum and low output power variation acquired in this laser are attributed to the self-averaging of multi-wavelength optical power induced by gain saturation and four-wave mixing ( FWM ) in SOA under the low cavity loss condition. The decrease of the spectral bandwidth and center wavelength with the decrease of driving current of SOA or the increase of cavity loss observed in the experiment was also discussed.