In the present article the concept of standard signal-to-noise ratio (StSNR) for THz time-domain spectroscopy was introduced, with which different systems could be evaluated quantitatively. Based on the characteristics of SNR, a brief method was given for acquiring StSNR. With this method for calculating the StSNR, the result data of a single test was the only item needed, which eased the users lot to compare the capability of various THz time-domain spectrometers. According to this method, a number of THz-TDS systems applied in various THz research groups were evaluated.