A high-energy 100-Hz optical parametric oscillator (OPO) based on a confocal unstable resonator with Gaussian reflectivity mirror (GRM) was demonstrated. A KTA-based OPO with a good beam quality was obtained when the magnifications factor was 1.5, corresponding to the maximum signal (1.53 μm) energy of 56 mJ and idler (3.47 μm) energy of 20 mJ, respectively. The beam quality factors (M2) were measured to be M2 x=5.7, M2 y=5.9 for signal and M2 x=8.4, M2 y=8.1 for idler accordingly. The experimental results indicated that the beam quality positively changed with the increase of magnifications factors, accompanied by an acceptable loss of pulse energy.