We realized a 85mm short cavity Fabry-Perot optical parametric oscillator by using high performance reflection mirror, and the experimental results of the THz wave output with this 85mm short cavity parametric oscillator based on the MgO:LiNbO3 Crystal are reported. Compared with the conventional 160mm parametric oscillator, the threshold decreases by 22.3%, the peak energy output of THz wave increases by 170%, the frequency tuning range increases from 0.5—2.4THz to 0.8—3.0THz. The THz wave linewidth was measured by using a Fabry-Perot interferometer based on the metal mesh plate.