Pupil size is commonly caused by the influence of external light, drug factors, brain stem lesions, poisoning and other diseases. 1. External light influence: from the darker environment into the bright light environment, the pupil will be smaller, this is the body’s normal reaction. 2. Drug factors, after taking benzodiazepine sedative drugs, the use of wool fruit rutabaga eye drops and other drugs, the pupil will appear to narrow the performance. 3. Brain stem lesions, pupil shrinkage is often seen in brain stem lesions, such as trauma leading to brain stem injury, hemorrhage leading to brain stem compression, brain stem infarction, brain stem hemorrhage and so on. 4. Poisoning, when there is morphine, organophosphorus pesticides and other drugs poisoning, there will be a pinpoint pupil, that is, the pupil shrinkage performance. 5. Other diseases, when there is coma, Horner’s syndrome, syphilis and other diseases, there will also be a narrowing of the pupil performance. When the pupil becomes small with obvious discomfort, it is recommended to consult a doctor in time to clarify the cause of the disease and carry out targeted treatment, so as to avoid delaying the condition.