The most common cause of precocious puberty in a 3-year-old boy with a beard is central precocious puberty, peripheral precocious puberty, and partial precocious puberty. 1. Central precocious puberty: (1) idiopathic precocious puberty: due to the hypothalamus negative feedback sensitivity to sex hormones decreases, gonadotropin-releasing hormone prematurely increased secretion. (2) Secondary precocious puberty: mostly seen in central nervous system abnormalities, such as tumors or occupying lesions in the cranial brain area, central nervous system infections, congenital developmental abnormalities, and acquired injuries, for example, hypothalamic malformation tumors, cysts, intracranial germ cell tumors, encephalitis, traumatic injuries, postoperative period, radiotherapy or chemotherapy, congenital hydrocephalus, and optic septal hypoplasia. (3) Other diseases: for example, a few untreated primary hypothyroidism. 2. Peripheral precocious puberty: the causes of precocious puberty include gonadal tumors, such as testicular mesenchymal stromal cell tumor; adrenal diseases, such as congenital adrenal hyperplasia; and exogenous factors, such as prolonged intake of estrogen-containing foods. 3. Partial precocious puberty: also called incomplete precocious puberty, due to partial activation of the hypothalamic-pituitary-gonadal axis. 3-year-old boy has a beard, there may be other reasons, such as local external use of drugs containing glucocorticoids, etc., it is recommended to go to the hospital in time to improve the examination to clarify the cause of the disease, under the guidance of the doctor to give targeted treatment or treatment.