The decrease of vestibular function is mainly due to the organic changes of vestibular and surrounding tissues, resulting in the disorder of balance function elements. It is commonly seen in cochlear vestibular diseases and nerve center lesions. However, vestibular function impairment caused by radiotherapy is rarely reported. This paper reports a case of vestibular function impairment after radiotherapy in a patient with brain metastasis of lung cancer, who developed dizziness 2 months after the completion of whole brain radiotherapy. The patient had walking instability, vestibular ataxia, limb weakness, and progressive aggravation, accompanied by tinnitus, hearing loss. With ameliorating microcirculation and vestibular rehabilitation training, the patient’s dizziness symptoms were improved while the postular instability continued to exist. High dose of radiation can make vestibular hair cells atrophy, vestibular capillary brittle idiopathic changes, resulting in the loss of semircular canalcysts and ampullae of nerve sensory epithelium, and vestibular hair cells may lose medullary nerve fibers. More oxygen free radicals, excessive oxygen free radicals during radiotherapy may cause hair cell lipid peroxidation or vascular degeneration, microcirculatory element disorder, damaging the elliptic cyst and balloon plaque supporting cells and hair loss of cells, vascular damage, and vestibular dysfunction.