A widely tunable mid-infrared generation in a quasi-phase matched optical parametric oscillator with a periodically polarized MgO-doped lithium niobate crystal is reported. Because of the lower coercive force and high damage threshold, the crystal is chosen and designed for six period patterns. In the quasi-phase matched optical parametric oscillator, the broadly tunable mid-infrared wavelength is realized by translating the position of the pattern in the nonlinear crystal without realignment adjustment. The experimental results show that the idler light lies in 3.649～4.115 μm when the pump wavelength is 1.064 μm. And the grating period of the crystal and the threshold pump energy of the optical parametric oscillator in this experiment are nearly consistent with the corresponding theoretical value, respectively.