The freeze-drying principle is basically carried out below 0 ℃, that is, in the frozen state of the product. The analytical drying is generally not more than 60 ℃. Under vacuum conditions, when the water vapor is sublimated directly, the drug remains in the frozen ice shelf, forming a spongy porous structure, so its volume is almost unchanged after drying. Before reuse, as long as water for injection is added, it will dissolve immediately again.