The THz wave output with the 85 mm F-P single cavity optical parametric oscillator based on the MgO∶LiNbO3 crystal was reported. The Nd∶YAG Q-switched laser with 1064 nm was used as the pump source, and the THz wave was tuned by changing the incident angle as well as the phase matching condition. THz wave frequency can be detected with the THz interferometer and the Si bolometer, or by detecting the idle light frequency. Experimental results indicate the THz wave is 0.9～3.0 THz tunable. The peak energy output is 102.5 pJ/pulse at 1.2 THz with the pump energy of 20 mJ/pulse, pulse width of 16 ns, repetition rate of 50 Hz. THz energy output was increased when a Si-prism array was introduced to decrease THz wave's total reflection at the interface of crystal. The THz wave beam pattern along the horizontal was measured with a metal-slit-detector. The influence of diffraction effect of the Si prism array on the THz wave beam spatial profile was discussed.