Abstract: Using the sounding data at Changchun Station in January and May of 2018, theoretical analysis and case calculations are carried to discuss how to properly apply the saturated vapor pressure formulas to water and to ice when the temperature is lower than 0 ℃ under different conditions. The saturation ratio of the actual atmosphere on May 29, 2018 is calculated. The saturation state and particle phase of the cloud corresponding to different saturation ratios are analyzed. The results show that: according to the "Regular High Air Image Observing Service Specification", data calculation and physical meaning, the precipitable water can be calculated by only the saturated vapor pressure formula with respect to water; To discuss the formation, growth of cloud particles and microscopic physical processes of precipitation, it is necessary to consider which saturated vapor pressure formula should be applied according to the phase of the particles and the surrounding environment in which the particles are located; The calculation of sounding data shows that there is coexistence of ice and water in the actual atmosphere, where it is saturated with respect to water and supersaturated with respect to ice; there is an environment where is unsaturated with respect to water and is supersaturated with respect to ice at 400 hPa in actual atmosphere.