The experimental results of high-efficiency 2.7 μm laser are demonstrated on quasi-phase-matched single-resonated optical parametric oscillator(OPO) in periodically poled lithium niobate with MgO(PPMgLN) pumped by 1064 nm laser of elliptical beam. Theoretical analyses of PPMgLN grating period tuning is presented. When grating period is 31.3 μm, a PPMgLN OPO could get output at wavelength of 2.7 μm. The pump beam polarization matches the e→e＋e interaction in PPMgLN (5 mol% MgO-doping), thus maximal nonlinear coefficient d₃₃(27.4 pm/V) is effective and walk-off of beams can be avoided. When pump power was 78 W with 8 kHz Q-switch, an average output power of 11.8 W at 2.72 μm was obtained with slope efficiency of 19.5%, with 24 W at the corresponding idle wavelength of 1.75 μm. The M² factor of 2.7 μm laser was 2.04, 5.56 in parellel and perpendicular direction respectively.