Pain is the main factor endangering human health at present. Objective, timely and effective pain evaluation is an important premise of pain treatment. Functional near-infrared spectroscopy (fNIRS) is a technique to measure cerebral cortex hemodynamics based on the neurovascular coupling mechanism. It has the advantages of sensitivity, noninvasive, continuity, objectivity, and portability. By summarizing the pathophysiological mechanism of pain and its characteristic cerebral cortical response, this paper briefly describes the mechanism of fNIRS in pain assessment, discusses the research progress of fNIRS in the diagnosis and treatment of acute and chronic pain in adults, and discusses the unique value of fNIRS in pain assessment of children, patients under general anesthesia and other special groups. It has great potential to optimize the evaluation and management of clinical pain in order to expand the thinking of clinical pain evaluation.