High pulse energy mid-infrared laser has important applications in remote atmospheric detection and object recognition. To obtain high pulse energy mid-infrared laser radiation, a self-grown ZnGeP2 (ZGP) crystal is employed as the nonlinear crystal of optical parametric oscillator, and the cutting angle is 55° with the type Ⅰ phase matching. A self-developed 2.09 μm La3Ga5SiO14 (LGS) electro-optically Q-switched Cr, Tm, Ho:YAG laser is used to pump the ZGP optical parametric oscillator (OPO) directly. The OPO conversion efficiency can be improved with a round-trip pumping way which can reflect pump light. The 2.09 μm Q-switched holmium laser radiation with pulse width of about 35 ns is used to pump the ZGP-OPO directly. Pulse energy of 5.9 mJ at 4.8 μm with an optical-optical conversion efficiency of 13.1% and a slop efficiency of 17% is obtained by the singly resonate operation; pulse energy of 9 mJ at 3.7 μm and 4.8 μm with an optical-optical conversion efficiency of 23.9% and a slop efficiency of 26.7% is obtained by the doubly resonate operation.