Both theoretical and experimental thermal quantitaive analyses of the precooling process of the sample before being plunged into the cryogen are carried out. The static temperature profiles in the cold gas layer above the cryogen is measured. The effects on the precooling process of the thickness of the cold gas layer, the plunge velocity, the radius of the samples and the initial temperature are investigated. The methods to avoid or relieve the harmful effects of precooling are proposed. In point of bio-electron-microscopy ultrastructure, the effect of the specimen precooling on cryofixation quility is also discussed.