Compact,efficient,widely-tunable coherent terahertz (THz) sources provide us convenient facilities in frequency domain spectroscopy and imaging.To satisfy such requirements,various methods have been exploited based on difference frequency generation (DFG) and THz parametric oscillation in nonlinear crystals.In this paper,theoretical and experimental studies of the DFG process system based on a walk-off compensated intracavity pumped dual-wavelength KTP OPO employing two identical KTP crystals and surface-emitted THz-wave parametric oscillator system with MgO:LiNbO3 crystal were presented.For DFG system,THz tuning range is from 0.2THz to 3.7 THz,with the maximum peak power of 11W at 1.68THz was obtained.For TPO system,THz tunable range from 0.8 to 2.9 THz with the maximum THz-wave output was 289.9 n J/pulse at 1.94 THz when pump power density was 211 MW/cm2,and the energy conversion efficiency was 3.43x10-6.The stability of our THz-wave parametric oscillator was about 7% when pump power density was 211 MW/cm2.