To explore the causes of the differences in water vapor pressure measurements between manual and automatic weather stations, based on the data of hourly air pressure, dry bulb temperature, and wet bulb temperature from 134 national reference climate stations, by using the water vapor pressure formulas on manual observation weather station and automatic weather station respectively, the regularities of water vapor pressure variation are studied corresponding to the increments of the station air pressure, dry bulb temperature, wet bulb temperature, automatic weather station temperature, and relative humidity. Through analyzing the meteorological observation data, the causes of the differences in water vapor pressure measurments between manual and automatic weather stations are discussed. Through the simulation and analysis of actual observation data, it is found that the influence of the systematic errors of air pressure and dry bulb temperature on the difference in water vapor pressure measurments between manual and automatic weather stations is very small, while the influences of the relative humidity and temperature of automatic weather stations are significant, especially relative humidity. For the given systematic errors of relative humidity and temperature of automatic weather stations, the variation range of the differences in water vapor pressure measurments varies with temperature, the higher the temperature, the greater the differences of water vapor pressure.