The basic principle of freeze-drying is based on the change of three states of water. Water has solid state, liquid state and gas state, and the three states can both transform and coexist. To obtain high-quality products, we should have a comprehensive understanding of the theory and technology of freeze-drying. The freeze-drying process includes three stages: pre freezing, sublimation and re freeze-drying. Reasonable and effective shortening of freeze-drying cycle has obvious economic value in industrial production.