Frailty syndrome is the core of the comprehensive geriatric assessment of the elderly, which affects the prognosis of elderly critical illness patients and becomes the hotspot of the current geriatric medical research of elderly patients. In critically ill elderly patients, the incidence rate of frailty syndrome is 21％-59％. Frailty syndrome is an independent risk factor in elderly patients with complications, short-term and long-term mortality. Moreover frailty is always accompanied by poor state and affects the health quality of these patients. In the field of critical care medicine in our country, the study of the frailty syndrome is still in its infancy. This article focuses on the research progress of frailty syndrome, and the assessment of the frailty critical illness elderly patients is helpful for the clinical doctors to determine the prognosis and treatment decision.