Abstract: In order to obtain narrow pulse width and high peak power laser output steadily in 2.12 m, the experiments of optical parametric oscillator (OPO) were devised, which were based on the fundamental beam of 1.06 m produced by side-pumping Nd∶YAG Zig-Zag slab, furthermore, the 1.06 m beam pumped KTP crystals by type II phase matching technology for producing 2.12 m. External OPO and intra-cavity OPO experiments were researched with the electricaloptical conversion efficiency at 1.53%, 2.86% respectively. At the frequency of 20 Hz, the energy of the 2.12 m laser output surpassed 70 mJ in intra-cavity OPO experiment while the pulse width and stabilization of energy were at 7 ns～9 ns, 8% approximately.