A compact, acousto-optic Q-switched, high power 4.1 μm mid-infrared intra-cavity periodically poled lithium niobate crystal doped with magnesium oxide optically oscillating laser system was reported. Based on the dynamics model of the intra-cavity single resonant optical parametric oscillator, the overthreshold multiple of the intra-cavity optical parametric oscillator and the down-conversion efficiency of the optical parametric oscillator under high power was analyzed. The method of adjusting the threshold of optical parametric oscillation in the experiment was clarified and the down-conversion efficiency of optical parametric oscillation under high power was optimized. The introduction of in-band pumping and single-ended bonded crystal improved the thermal stability at high power pumping. Based on the optical field propagation theory and the resonant cavity stability theory, and considering the thermal effect of the gain medium, the three-wave fundamental mode pattern matching of fundamental frequency light, signal light and idle frequency light in the cavity was numerically simulated when the high-power pump was injected, in order to ensure the stable operation of the optical parametric oscillator at high power. The down-conversion efficiency of parametric laser was improved by adjusting the threshold of optical parametric oscillation. Finally, the output of mid-infrared high pulse repetition frequency watt-level laser at 4.125 μm was obtained, the laser repetition frequency were adjustable from 1 to 100 kHz, the pulse width was less than 9 ns, the maximum single-pulse energy was 36.7 μJ, the maximum peak power was 4.257 kW, the maximum output power was 1.12 W, the corresponding to down-conversion efficiency was 29.7%, and optical-optical conversion efficiency was 4.26%.