The THz source based on the nonlinear optical effects possesses the special properties and advantages. A theoretical study has been made for the generation of widely tunable narrow-band THz wave based on the process of the difference frequency generation (DFG) and the stimulated polariton scattering in a GaSe crystal and a MgO doped LiNbO3 crystal, respectively. The relations among the THz wave, pump wave, idler wave and phase-matching angle are analysed. The condition and range of THz radiation generation are given. A NdYAG laser and an optical parametric oscillator (OPO) have been used to provide the pump and idler beams. A THz radiation generation system is built under collinear phase-matching conditions. A THz-wave parametric generator (TPG) is developed, and the TPG emits THz efficiently.