Precipitation is likely to take place as precipitable water vapor (PWV) of the atmosphere reaches a certain value. Thus, the threshold of PWV (PWVt), which can be regarded as the research bridge between PWV and precipitation. In the aim of calculating PWVt precisely, this paper has brought in “precipitable water vapor saturation (PWVsat)” of the atmosphere and deduced the computing formula of PWVsat under various atmosphere conditions. From the formula, it is shown that PWVsat is a function of surface temperature (ts), which indicates the maximum water vapor that a bottom-up saturated atmosphere is capable to hold. The atmosphere needs to be wet enough to form clouds and rains. Therefore, it is inferred that ts is a factor in PWVt. To prove that, PWVt and ts of 36 gauge stations ranging from May 2015 to October 2016 are studied and 1122 precipitation cases are selected and fitted. The results show that the fitting formula of PWVt and computing formula of PWVsat are highly alike, proving the 2 variables are closely correlated. Meanwhile, fitting parameters of each of 36 stations are given for relating PWV to precipitation. This method has gone through statistical examination to find that its accuracy rate, missing rate and vacancy rate to be 93.69%, 2.32% and 3.99% respectively in precipitation prediction, which denotes a sound application value in predicting rainfall. Finally, the method is applied in a rainfall case to show that PWVt can predict precipitation well, and the difference between PWV and PWVt is correlated to precipitation amount.