Using Nd∶YAG laser pump intra-cavity optical parametric oscillator, dual wavelength near 2 μm can be generated.Through collinear difference-frequency-generation, widely tunable and continuous Mid-infrared- radiation in the range of 8~19 μm is obtained under the condition of type I phase matching. The maximum output power of 7.4 kW (pulse energy of 33.66 μJ) is achieved at 8.76 μm, corresponding to photon conversion efficiency about 4.43%. The reasons that affecting conversion efficiency are analyzed and new measures are proposed to compensate walk off angle.