The dilated pupil on one side can be caused by eye diseases or central nervous system diseases, as follows: 1. Eye diseases: including eye trauma, injury to the oculomotor nerve, and possibly eye diseases such as glaucoma or ocular nerve diseases; 2. Central nervous system diseases: commonly, brain trauma affecting the eye or brain hemorrhage and brain infarction in the central nervous system leading to brain herniation, which can also Unilateral pupil dilatation can occur. Both of these problems require a visit to the hospital, where a specialist can determine which problem is which and then actively treat the symptoms.