The main difference between isotonic hypertonicity and hypotonicity is the difference in osmolality and the use of the corresponding solutions. Isotonic means that the osmolality is equal to the plasma osmolality (about 300mOsm/(kg-H2O)), such as 0.9% NaCl solution is an isotonic solution, in which red blood cells can maintain their normal shape and size; whereas osmolality higher than that of plasma is hypertonic, in which red blood cells will be dehydrated and crumpled; lower than the plasma is hypotonic, in which red blood cells will be edematous and even ruptured. Isotonic solution is mainly used for isotonic dehydration caused by general infusion, diarrhea, massive vomiting, etc. Hypertonic solution is mainly used for hypotonic dehydration that only replenishes water without replenishing electrolytes after dehydration, and hypotonic solution is mainly used for hypertonic dehydration caused by uremic syndrome and massive sweating, etc. Isotonic, hypotonic and hypertonic mainly use plasma osmolality as a criterion to judge the level of osmolality, which has an important role in clinical practice, and the solutions with corresponding osmolality should be used after careful comparison of the indications.