There are many causes of pupillary dilation, including ocular diseases, drug effects and intracranial lesions, and whether the pupil can be resuscitated is mainly related to the cause. The pupil is a small round hole in the middle of the iris, with a normal diameter of 2.5-4 mm. Its function of dilatation and reduction is governed by the pupillary sphincter and pupillary dilator muscles. Normally, the pupil will narrow when light is enhanced and dilate when light is diminished. Pupil dilation may occur if the patient uses certain medications that dilate the pupil, such as atropine and compound tropicamide drops; some intraocular pathologies such as glaucoma and iris atrophy can also cause pupil dilation, and such patients usually do not have life-threatening conditions. If the pupil is dilated due to central nervous system injury, such as acute cerebral hemorrhage and traumatic brain injury, it is usually more serious. If timely surgery is performed to remove the hematoma and prevent brain herniation from occurring, there may be hope for salvage, but serious sequelae are usually left behind. If the pupil is dilated to the edge of the iris and the reflex to light disappears at the end stage of resuscitation in a critically ill patient, it may be a sign of death. Therefore, there are more causes of pupil dilation that need to be identified.