The continuous-wave Mid-Infrared Radiation(Mid-IR) was experimentally obtained by difference frequency quasi-phase-matching in a MgO-doped periodically poled LiNbO3 crystal (MgO∶PPLN), which the narrow linewidth light sources with 1 083 nm and 1 550 nm were used as pump light and signal light,respectively. Moreover, the multiple mid-IR wavelengths were realized by adjusting the signal wavelength and using the temperature controlling on a MgO∶PPLN. The wavelength tuning region is around 3 547.6 nm to 3 629.1 nm. A maximum mid-infrared radiation power of 3.2 mW at 3 597.0 nm is generated when the optical power of signal and pump lights are amplified to 3.5 W and 2.8 W respectively. The power jitter of mid-infrared output is less than ±1.6% after along time test recording. This study can be used as a reference for the design and development of narrow line width multi-wavelength continuous-wave infrared light source.