To assess the potential applications of terahertz waves in space communication, radar and remote sensing, the transmission characteristics of terahertz waves in atmosphere should be studied. As the transmission attenuation of terahertz wave in the range from 0.1 to 2 THz are caused mainly by water vapor, the THz Time-Domain Spectroscopy (THz-TDS) is used to measure the transmission spectra of the air with different humidity in a 2 m long THz transmission path. The absorption spectra, transmission rate and weak attenuation widows of water vapor in the frequency range from 0.1 to 2 THz are obtained.