Terahertz (THz) detection is a novel and very effective coherent detection method,and both abundant amplitude information and ample phase information of materials which need to be detected and consequently contrast can be achieved.The THz spectral properties of several kinds of water are investigated based on the terahertz time domain spectroscopy (THZ-TDS) system.A sort of iterative approach is given,in order to avoid the result error caused by weak absorption approximation,which improves the traditional methods of data processing of transmission spectroscopy of terahertz time-domain.Compared with different filters,Symlets9 wavelet filter is considered to be optimal filter.The exponential change laws of refractive index and absorption coefficient with frequency are gotten with square of correlation coefficient up to 0.9 upwards.The results indicate that both square of correlation coefficient and fitting curve parameters of different kinds of water are different,which can be defined as the criteria to distinguish water quality.