A Tunable Mid-IR dual frequency laser based on single-resonant optical parametric oscillator is presented. The mid-infrared wavelength tunability of 3~3.8 μm is achieved by adjusting the temperature and polarization period of MgO:PPLN crystal. When the pump power is 6.9 W and the temperature of the MgO:PPLN is 75℃, output power of 1.25 W is obtained. The highest optical-optical conversion efficiency of the idler light with respect to pump light is 18.2%. The beat note frequency is equal to the beat note frequency of the pump, which can be tuned from 125 to 175 MHz. The modulation index of the beat note of idle could be adjusted with different dual-frequency power radio of pump.