A high efficient tunable mid-infrared Optical Parametric Oscillator (OPO) based on a periodically poled MgO-doped lithium niobate (PPMgLN) crystal is reported. By using a self-designed Laser Diode (LD) double-end-pumped optoacoustic Q-switched Nd:YVO4 laser as the pumping source of the OPO, the quasi-phase matched single resonant mid-infrared laser output is achieved. When the pumping source has its power of 14.37 W, the idler light output with an average power of 2.39 W is obtained. Its conversion efficiency is 16.63%. By adjusting the period of the PPMgLN crystal, the wavelength of the idler light can be tuned in the range from 3.41 μm to 3.97 μm.