Vacuum freeze dryer, like other drying methods, must meet two basic conditions to keep sublimation dry, namely, the continuous supply of heat and the continuous elimination of steam generation. In the beginning stage, if the material temperature is relatively high, the latent heat needed for sublimation can be obtained from the sensible heat of the material itself. But with the sublimation, the material temperature will soon drop to the temperature balanced with the vapor partial pressure in the drying room. At this time, if there is no external heating, sublimation drying will stop. In the case of external heating, if the steam generated by sublimation is not eliminated in time, the steam partial pressure will rise, and the material temperature will rise accordingly. When the freezing point of the material is reached, the ice crystal in the material will melt and the freeze-drying will not be carried out.