Abstract: Terahertz ray, one kind of electromagnetic radiation with ultrabroad bandwidth of spectrum, is generated by triggering a photoconductive emitter with femtosecond laser pulses; and it has many important scientific, industrial and military applications. Generation and detection of terahertz ray are analyzed, and several applications including THz-TDS (terahertz time-domain spectroscopy),two-dimensional electro-optic imaging of THz beams and THz ultra-wideband radar are also reviewed.