Asymmetry of Bell’s phenomenon in both eyes is one of the symptoms of monocular double supination palsy. Monocular double supination palsy is manifested by asymmetry of Bell phenomenon in both eyes, which is often poor or absent in the affected eye. Double supination palsy in one eye means that the superior rectus and inferior oblique muscles of one eye are paralyzed at the same time. The clinical presentation is a downward strabismus of the affected eye, mostly combined with horizontal strabismus and ptosis, which is mostly pseudo or mixed and easily misdiagnosed as congenital ptosis. What are the preventive efforts associated with Bell’s phenomenon asymmetry in both eyes? The vertical slope is greater in the first eye position in monocular double supination palsy, which can be accompanied by overfunction of the antagonist muscle in the ipsilateral eye and the spouse muscle in the contralateral eye, so surgery should focus on weakening the antagonist and/or spouse muscles to correct the vertical slope. If the affected eye is a gaze eye, according to Hering’s rule, the upward slope of the healthy eye is obvious, so we can consider weakening the upward and downward slope muscles of the healthy eye, which can take care of the frontal and lower gaze field without double vision. It is thought that this may be a residual partial paralysis of the motoneural nerve during the recovery process. In terms of congenital origin, according to the anatomy of the motoneuron, the nuclei of the motoneuron are, from top to bottom, the nucleus of the levator aponeurosis, the nucleus of the superior rectus and the nucleus of the inferior oblique, with the nucleus of the internal rectus and the nucleus of the inferior rectus. The nucleus accumbens and the nucleus accumbens may be damaged during maternal pregnancy. In acquired cases, trauma, brain inflammation, tumors, and other factors may lead to actinic nerve palsy. In both congenital and acquired cases, there is a certain order of recovery from the nerve palsy, with the superior levator muscle recovering the earliest and fastest, followed by the medial rectus, inferior rectus, and pupillary sphincter, and the superior rectus and inferior oblique muscles recovering the latest or not at all, thus showing the characteristic double supination muscle palsy in one eye. In true severe ptosis, Bell’s phenomenon improves after correction of the eye position, and for cosmetic reasons shortening of the levator muscle or suspension of the frontalis muscle flap is feasible but the amount of surgery should be such that it does not cause exposure.