A scheme to generate high-efficiency terahertz (THz) wave based on cascaded difference frequency generated in a periodic polarized lithium niobate (PPLN) crystal with an optical parametric oscillator is presented. By optimizing the reflectivity of the resonator mirror, Stokes photons oscillate in the cavity, while anti Stokes photons pass through the cavity only once. All the Stokes photons propagate from the left side of the cavity to the right, which ensures that all the Stokes photons pass through the PPLN crystal in one direction. Every time the Stokes photons oscillate in the cavity, the photons of the pump waves are transferred to the Stokes photons. The Stokes processes are enhanced and the anti-Stokes processes are depressed simultaneously. The energy conversion efficiency from pump wave to THz wave reaches 21.2% with pump wave intensity of 10 MW/cm2 and signal wave intensity of 0.01 MW/cm2 at 100 K operation temperature. The above efficiency exceeds the Manley-Rowe relation, which provides an optimal scheme for generation of high efficient pulse, quasi-continuous, continuous THz waves.