Insulin is an important method for diabetic patients to control blood glucose, the common injection sites of insulin include buttocks, upper arm, abdomen, outer thighs, etc. The abdomen absorbs the fastest, followed by the upper arm, and the outer thighs and buttocks absorb at a similar rate. As for which part of insulin injection is the best, it needs to be decided according to the type of insulin, blood glucose, thickness of patient’s subcutaneous fat, etc. It is divided into the following cases: I. Pre-meal insulin needs to reach the peak value as soon as possible after subcutaneous injection in order to control postprandial glucose, and the fastest absorbing part of insulin is the abdomen, so the pre-meal insulin is recommended to be injected in the abdominal area subcutaneously. Second, intermediate-acting insulin and long-acting insulin analogs belong to basal insulin, which is mainly used to control fasting blood glucose, and can be injected subcutaneously in the buttocks or outer thighs to achieve smooth absorption. Third, thin patients are not recommended to inject in the outer thighs or upper arms, as too little subcutaneous fat can easily cause insulin to be injected into the muscle layer, resulting in blood glucose fluctuations. Regardless of the injection site needs to be replaced regularly to avoid the formation of hard knots at the injection site or subcutaneous fat atrophy.