An all-solid-state mid-infrared optical parametric oscillator with wide tunability by using two identical multi-grating periodically poled 5-mol.-% MgO-doped lithium niobate cascaded is reported. The pump laser is an acousto-optically Q-switched Nd:YAG laser with 150-ns pulse width at repetition rate of 10 kHz. Wide tunability from 2.789 to 4.957 \mu m at the idler wavelength is achieved by varying the temperature from 40 to 200 ℃ and translating the grating periods from 26 to 31 \mu m with a step of 0.5 \mu m. When the incident pump average power is 8.15 W, the maximum idler output average power is 2.23 W at 3.344 \mu m and the optic-optic conversion efficiency is about 27.4%.