According to statistics, spasticity occurs in 39% of stroke patients and 50% of patients with traumatic brain injury. Persistent spasticity not only leads to muscle contracture and pain, but also seriously affects the functional recovery of the limb, resulting in decreased ability to perform activities of daily living and quality of life. At present, commonly used treatments to relieve muscle spasm include physical therapy, oral or intrathecal injection of anti-spasticity drugs, botulinum toxin injection, pharmacological nerve block, and surgery, etc. Some of these measures have obvious side effects (drowsiness, psychiatric symptoms, etc.) and limited efficacy, and some are invasive operations, so their use in clinical practice is somewhat limited. In clinical practice, the search for non-invasive, less adverse and effective antispasticity measures has been the goal of rehabilitation medicine practitioners. Transcranial direct current stimulation (tDCS) is a non-invasive technique that uses a weak current (1 to 2 mA) to regulate the activity of neurons in the cerebral cortex. Stimulation of the primary motor cortex and its surrounding areas of the affected upper limb with tDCS cathode resulted in a significant suppression of muscle spasm in the affected upper limb. tDCS suppresses the primary motor cortex and its surrounding areas of the affected upper limb, probably by inhibiting the over-activation of S1M1, thus changing the imbalance between the central inhibitory system and the eccentric system, and achieving the suppression of muscle spasm in the affected upper limb. tDCS can alleviate the upper limb spasm after stroke, and together with proper rehabilitation, it can improve the limb motor function and enhance the patient’s ability to perform daily life.