Recently, due to the potential applications, the developing of compact, portable, low cost, and tunable THz wave became a research focus of THz technology. The light source with a wavelength range from 600—2000 nm frequently serves as a pumped source of THz wave due to its property of compact size, low cost, tunable frequency and stable operation. In this paper, tunable dual-wavelength continuous-wave pumping source and THz generator with an internal-cavity optical parameter oscillator are overviewed and analyzed in detail. It is shown that pumping source in the near-infrared wave band is more advantageous than those in the mid-infrared wave band for compactness in size. It is also shown that the difference frequency and nonlinear cascade processes have become an important way to generate THz radiation for wide tuning merit in such a system.