The energy level diagram and absorption spectrum of Tm:YAP crystal and phase-matching properties of ZeGeP crystal are analyzed, and a middle-infrared optical parametric oscillator (OPO) under room temperature is presented. The wavelength of laser is 3.88, 4.00 and 4.14 μm, and the output power is 7.16 W at 10 kHz, the corresponding optical-optical conversion efficiency is up to 49.4%, and the slope efficiency is 48.9%. The single pulse width is 81.47 ns, the energy of single pulse is 0.71 mJ, the peak power of single pulse is 8.78 kW, the quality of middle infrared laser beam is 3.8(horizontal) and 4.0(vertical).