In order to design high-power narrow linewidth 4.3 μm laser, the narrow linewidth 4.3 μm laser was obtained by ZnGeP₂ OPO (optical parametric oscillation) pumped by a 2.7 μm laser. Experimental results were analyzed in detail. The pump source was a 2.7 μm KTiOPO₄ OPO laser pumped by a 1064 nm Nd:YAG laser, and the KTiOPO₄ OPO was singly resonated. Two identical KTiOPO₄ crystals were placed with opposite optical axis to compensate the walk off effect. The KTiOPO₄ crystals were cut asΦ=0°,θ=62° to generate 2.7 μm wave band laser. To utilize the large effective nonlinear coefficient, the phase matching was based on type Ⅱ(B) (o→o+e). The ZnGeP₂ OPO was singly resonated too. To output narrow linewidth laser, the phase matching is based on typeⅡ(B) (o→o+e).The ZnGeP₂ crystal was cut as Φ=0°,θ=68° to generate 4.3 μm laser. The energy of the 2.7 μm laser is 7.5 mJ with a pulse of 8.6 ns and line-width 12 nm.The pulse energy of 2.12 mJ at 4.26 μm with an optical-optical conversion efficiency of 28.3% and a slop efficiency of 32.6% is obtained, and the pulse width is 8.7 ns, the line-width is 30 nm. The beam qualities M² of horizontal and vertical directions are 6.2 and 13.5, respectively.