This thesis utilized pulse shaping and Gerberg-Saxton algorithm to develop high power tunable narrowband terahertz radiation and applications. By the way of using Freezing algorithm and Gerberg-Saxton, pulse width, pulse duration, pulse position and pulse number can be controlled. The pulse train can be pumping source of high power tunable narrowband terahertz radiation, by avoid the saturations and different pulse duration. The simulation shown the result can have 5GHz narrowband width closing to commercial product. Besides, we demonstrated the THz multi-wavelength interference method by using THz-TDS (terahertz time domain spectroscopy) to measure the liquid crystal cell gap. We have also used THz-TDS to investigate the complex optical constants and birefringence of cholesteric liquid crystal mixture BL006. The extraordinary (ne) and ordinary (no) indices of refraction at are 1.784-1.817 and 1.516-1.555 giving rise to a birefringence of 0.25¡V0.29 in 0.2THz~1.6THz.