Far infrared (FIR), an invisible, non-ionizing radiation of electromagnetic wave, is widely used in medical because of the absorption by body easy and lots of beneficial effects for the human being. Of the numerous effects in FIR, one can promote the human microcirculation and increase secretion in neuroendocrine substance of the opium peptide, which has sedative and analgesic effects for the human body.The analgesic principle by FIR can be divided into two categories: the thermal effect and non-thermal effect. The thermal effects can help the expansion of human blood vessels, improve the human peripheral circulation, and reduce pain caused by poor peripheral circulation. Both thermal and non-thermal effects relieve pain by promoting opium peptide secretion. FIR had been widely used in Chinese medical clinical but without a guideline of the analgesic function for the use. The article will discuss the principle of analgesic effect in the human body by FIR and be a reference for clinicians.