The methods to generate terahertz (THz) waves can be divided into electronics and photonics. We use nonlinear optical difference frequency generation to obtain THz waves which belongs to photonics method. It is portable and run at room temperature, with high power and low cost. A simultaneous dual-wavelength quasi-continuous wave (CW) laser-diode-end-pumped Nd:YAG laser polarized operating at 1319 nm and 1338 nm has been demonstrated and the laser at 1064 nm is suppressed successfully. A total polarized quasi-CW output power of 2.22 W is obtained at a pump power of 20 W with a pulse repetition rate of 50 kHz, and the slope efficiency is 12.72%. The corresponding pulse width is 71.91 ns. The beam quality factor M2 is as low as 1.165 and the instability of the output power is less than 0.487% at the polarized wave output power of 2.17 W. Using the simultaneous dual-wavelength laser, the highly coherent THz wave radiation of 3.23 THz will be generated by nonlinear optical difference frequency method in DAST (4-N,N-dimethylamino-4′-N′-methyl-stilbazolium tosylate) crystal. The ideal output peak power is 4.71 mW. The simultaneous dual-wavelength laser provides theoretically basis for generating highly coherent terahertz wave radiation by nonlinear optical difference frequency method.