We report a high-efficiency and high-power mid-IR laser generation by a periodically poled (PP) MgLN-based optical parametric oscillator (OPO) working at room temperature, from which $9.23\mathrm{W}$ of output was obtained at $3.82\mu \mathrm{m}$ when pumped by a linearly polarized acousto-optic Q-switched $\mathrm{Nd}:\mathrm{Y}\mathrm{V}{\mathrm{O}}_4$ laser of $48.2\mathrm{W}$ output. A semi-external-cavity structure was applied in the OPO system to avoid the conversion saturation caused by the thermal lens effect of the PPMgLN crystal and thus to maintain the high conversion efficiency and the high beam quality under the high power level. It was computed that 29.4% slope efficiency and 19.2% total efficiency were realized for the $1.063–3.82\mu \mathrm{m}$ conversion.