The mid-infrared (MIR) laser of 3-5 μm has low propagation loss in the atmosphere, which is located in the atmospheric transparency window, and contains many absorption spectral lines of molecules and atoms. It is also known as the "molecular fingerprint region". Therefore, mid-infrared laser in this wavelength range have important applications in many fields such as environmental monitoring, military, medical, and remote sensing. Currently, the main methods for generating MIR laser output include fiber lasers, quantum cascade lasers, transition metal ion-doped solid-state lasers, and optical parametric oscillator (OPO) based on nonlinear frequency conversion technology. Among them, OPO has many advantages such as compact structure, flexible tuning methods, and high output efficiency, which has become an important means for generating mid-infrared lasers. A nanosecond mid-infrared fan-out MgO-doped periodically poled lithium niobate (MgO: PPLN) OPO is studied with wide tunning range and high conversion efficiency. It is pumped by a 1 064 nm Q-switched laser.The entire system consisted of a pump source, mirrors, half-wave plate (HWP), polarizing beam splitter (PBS), optical isolator (ISO), lens, OPO resonant cavity, nonlinear crystal, and filters (Fig.1). The power and polarization of the pump were adjusted by the HWP and PBS. An optical isolator was used to prevent the reflection of pump wave back into the laser source to avoid damaging the source. The pump wave was then focused by the lens into the center of the MgO: PPLN crystal. Under high-power pumping conditions, the parametric light oscillated inside the cavity, and the output light was separated by a long-pass filter (LPF) with cut-off wavelength of 1 100 nm and a germanium (Ge) window.By reducing the repetition rate of the pump, the oscillation threshold of the OPO was effectively reduced. At the repetition rates of 10 kHz, 20 kHz, and 30 kHz of the pumping laser, the OPO oscillation thresholds were measured to be 0.4 W, 1 W, and 1.6 W, respectively. When the pumping power was 4.68 W and the poling period of MgO: PPLN was 30.47 μm, a maximum MIR laser output power of 0.833 W at 3.4 μm was obtained, corresponding to an optical-to-optical conversion efficiency of 17.8% (Fig.2). The poling periods of MgO: PPLN can be changed by shifting the crystal from 31.05 to 28.8 μm. This corresponds to the generation of a signal wave from 1 440.7 to 1 670.0 nm and an idler wave from 3 171.1 to 4 088.1 nm (Fig.3), respectively. The experimental results were in good agreement with the theoretical simulation values (Fig.4). Using a photodetector and an oscilloscope, the pulse widths of the pump and signal waves were measured to be 10.9 ns and 8.1 ns, respectively (Fig.5).A mid-infrared optical parametric oscillator based on a fan-out MgO: PPLN crystal was designed, which features a wide tuning range, high output efficiency, and narrow pulse width. At a pumping frequency of 10 kHz, the maximum output power of 3.4 μm mid-infrared laser was 0.833 W, with a pumping power of 4.68 W, and the corresponding optical-to-optical conversion efficiency was 17.8%. The output wavelengths at the different poling periods of MgO: PPLN were measured, which were well-matched with the theoretical values. By means of period tuning, signal light with wavelengths of 1 440.7-1 607.0 nm and idler light with wavelengths of 3 171.1-4 088.1 nm were obtained. And the FWHM pulse width of the signal light was ~8.1 ns. This experiment provides a feasible solution for developing compact, high-power, wide-tuning nanosecond mid-infrared laser sources.