We report a linear extra-cavity, tunable mid-infrared, Continuous Wave (CW) singly resonant Optical Parametric Oscillator (OPO) based on periodically poled MgO-doped LiNbO3 (PPMgLN). A Nd:YVO4 laser pumped by 808 nm LDA is used as the pump source. The threshold of OPO system is only 1.09 W at 1 064 nm. A maximum idle laser output power of 346.8 mW at 3.202 μm has been achieved when the pump power is 4.55 W and it corresponds to an (only idler) optic-optic conversion efficiency of 7.6%. By shifting the PPMgLN crystal, the periods of the domain structure on the PPMgLN wafer can be changed, thus enabling a mid-infrared spectral widely tuning range of the laser output from 2.98～4.19 μm. This laser is very compact, simple and operated at room temperature.“中红外波段全固态光学参量振荡器产品研究与开发”。