Operation characteristics of an injection-seeded terahertz parametric oscillator based on ring-cavity configuration are reported. The nonlinear crystal used in the experiment is MgO∶LiNbO3. The terahertz wave is emitted perpendicularly to the output surface. The Stokes ring-cavity is constituted by three mirrors and total reflection surface of MgO∶LiNbO3 crystal. The pumping source is a Q-switched laser, whose output wavelength is 1064.2 nm and pulse width is 7.5 ns. When the pumping pulse energy is 105.5 mJ and the transmittance of the Stokes wave output coupler is 20.1%, the maximum terahertz pulse energy output leads to the output voltage of 3.3 V, and the corresponding Stokes pulse energy is 16.1 mJ and the pumping energy threshold is about 10 mJ. Under the same pumping conditions, the output energy and pumping pulse energy threshold of the injection-seeded terahertz parametric oscillator based on ring-cavity configuration and injection-seeded terahertz parametric generator are compared with that of a ring-cavity terahertz parametric oscillator without seed injection. The results show that the injection-seeded ring-cavity terahertz parametric oscillator based on ring-cavity configuration has higher output energy and lower pumping energy threshold, and the oscillator has more obvious advantage in energy output when the pumping energy is smaller.