The condition and parameter design for an optical microcavity parametric oscillator formed by micro-flake GaAs crystals to generate terahertz (THz) wave were studied according to a cavity phase matching method. The optimum cavity length under the cavity phase matching condition was calculated. The output of THz wave was studied by adjusting the temperature of the GaAs crystal. The lowest energy thresholds at different wavelengths were simulated. The result showed that a wider tunable THz output range could be achieved more easily by using the cavity phase matching when the perfect phase-matching was difficult to realize. This result could provide experimental and theoretical reference for the development of miniature optical THz sources.