ZnGeP2 crystal with high nonlinear coefficient (d36=75 pm/V), broad infrared transmission region (0.7～12 μm) and very high thermal conductivity (0.18 W/(m?K)) is very suitable for high power mid-infrared radiation generation. Phase matching characteristics of ZnGeP2 optical parametric oscillator (OPO) are analyzed theoretically, the requirement of type Ⅰ phase matching angle of 52.5～55.2° must be met to achieve continuously tuning of 3～5 μm. Experimentally, the acousto-optic Q-switched Tm, Ho∶YLF laser end pumped by fiber-coupled diode is employed as the pumping source of OPO with the maximum average power of 4 W, pulse duration of less than 40 ns, and pulse repetition rate of 100～10000 Hz. For higher extraction efficiency and low threshold operation to the quasi-three level laser system, the Tm,Ho∶YLF crystal is cooled by liquid nitrogen reserved in a small detector type dewar, and works at the temperature of 77 K. The 15-mm-long, 55.7° cut ZnGeP2 as a frequency converting crystal was placed in a plano-plano cavity with resonator length of 20 mm. Under a pumping power of 3.6 W and pulse repetition rate of 10 kHz, the mid-infrared coherent radiation of OPO near 4.1 μm was achieved with output power of 0.7 W, pulse width of 20 ns, optical-optical conversion efficiency of 20% and threshold pump power of 0.65 W.