To achieve high-power mid-infrared laser output with high efficiency, the optical parametric oscillator (OPO) based on MgO: PPLN crystal is researched. The pump source is linearly polarized Yb-doped fiber laser (YDFL) with master oscillator power amplifier (MOPA) structure. Experimental results indicate that the YDFL can realize the pulse linearly polarized laser with the highest power of 79.1 W at 1064.1 nm. With the pump laser, the OPO obtains the maximum MIR power of 9.15 W at 3.754 µm by optimizing the curvature radius of output coupler and the pump waist diameter. The corresponding optical-to-optical conversion efficiency is 11.57%. The repetition rate and the pulse width of mid-infrared laser are 300 kHz and ～110 ns, respectively.