The weakness of the right hand cannot clench the fist. The causes of this phenomenon are the following: 1. The most common cause is cerebrovascular disease, which causes damage to the nerve cells associated with the left cerebral hemisphere due to cerebrovascular disease, resulting in weakness of the right hand and inability to clench the fist. 2. Other central neurological diseases, such as intracranial tumors, encephalitis, and multiple sclerosis, can also lead to the phenomenon of weakness of the right limb. 3, Peripheral nerve diseases, such as Guillain-Barré syndrome, 4. In addition, there are also myasthenia gravis, motor neuron disease, etc., which can lead to the phenomenon of right hand weakness. Therefore, if the above phenomenon occurs, it is important to go to the neurology department of a regular hospital and get a clear diagnosis by doing relevant tests.