A terahertz (THz)-wave parametric oscillator (TPO) pumped by a stable and single-longitudinal mode Q-switched Nd:YAG laser under various room temperatures is demonstrated. It is based on a cavity TPO architecture allowing stable single-resonance operation and low oscillation threshold. The output results, including the effects of the room temperature on this wavelength-agile TPO with a MgO:LiNbO3 crystal, indicate that the performance of TPO under the lower temperature is better. We obtain a widely tunable THz-wave source in the range 104 to 226 μm via tuning the cavity flexibly under different room temperatures. The peak power of the THz wave reaches 220 mW at the wavelength of 146.2 μm when the room temperature is 20°C. The peak power of the THz wave decreases to 48 mW when the room temperature rises from 20°C to 25°C.