The air water vapor content and annual precipitation in China decrease from southeast coast to northwest inland, and there is a good spatial similarity between them. By studying the fitting relationship between them, it is possible to find a good statistical law and a breakthrough in the study of influence of air water vapor content on annual average precipitation. Based on this spatial similarity, the average precipitation (P) and air water vapor content (W) of 121 sounding stations from 1971 to 2000 in China were studied and the fitting formula between them was found, namely P=44.385 (W-2.66), with a highly positive correlation between the two, R2=0.8293. Further, the study on monthly average precipitation and air water vapor content over the years found a high positive correlation between the two, and the above results passed the review and verification. Air water vapor content W multiplied by the study area is the stock of air water vapor, namely the liquid volume converted from air water vapor. There are many factors affecting annual average precipitation. The minor influencing factors were ignored and the main influencing factor was found in the fitting formula. It is the stock of air water vapor, and its quantitative parameter is the air water vapor content W. The study also found that when the air water vapor content is equal to or more than 14 mm, the annual average precipitation of all stations would be equal to or more than 400 mm. The annual average water vapor content equal to or more than 14 mm is a sufficient and unnecessary condition for the average annual precipitation equal to or more than 400 mm.