An efficient widely tunable terahertz(THz)-wave parametric oscillator (TPO) has been designed based on the basic principle of optical parametric oscillator. The design of TPO comprises the theoretical calculations for wavelength dependent refractive index dependent wavelength when the frequency is in the far infrared region, the low-loss parametric gain has been discussed as a same form as the parametric gain in the optical region, realized the non-collinear phase-matching con ditions of the terahertz optical parametric oscillator and structure of lithium niobate (LiNbO3) for TPO. The tunable spectrum range of terahertz-wave has been achieved. To increase THz output, a cut exit was made at the corner of the Li NbO3 crystal.