High-efficient THz-wave radiation is achieved from a Si-prism coupler THz-wave parametric oscillator via a block of MgO:LiNbO3.The THz-wave radiation from 0.95 THz to 2.10 THz is obtained.The average power of THz-wave is 580 nW and the output energy of Stokes wave is 2.7 mJ at 1.8 THz when the pump energy is 101 mJ,corresponding to the THz-wave energy conversion efficiency of 5.74×10-7.During the experiments,the first-order and the second-order Stokes waves are observed.The frequency shift of the first-order Stokes wave is equivalent to the frequency of the generated THz-wave.