A widely tunable mid-infrared optical parametric oscillator (OPO) with high peak power, nanosecond pulses output, which was based on MgO-doped periodically poled LiNbO₃ (MgO: PPLN) was reported in this paper. The MgO: PPLN OPO with a domain period of 29 μm was pumped by a fundamental laser of 1064 nm with high beam quality, and the fundamental laser operated at the repetition rates from 30 kHz to 50 kHz with the pulses duration less than 4 ns. The 48.45 kW peak power, 1.19 ns, and 30 kHz pulses at 3.93 μm were obtained, as the temperature of MgO: PPLN was at 80 ℃. The tunable range of mid-infrared wavelength was from 3.77 μm to 3.96 μm, as the temperature of MgO: PPLN was from 50 ℃ to 200 ℃. According to the experimental results, the characteristics of conversion efficiency and temperature tuning characteristics under different repetition rates were analyzed and discussed.