Frailty is a clinical state or syndrome that predispose the patients to adverse health outcomes due to the decline of physiological reserve and stress adaptability. No gold standard is available for the assessment of frailty, but the cumulative defect model has been found to be better at predicting various adverse clinical events than other frailty assessment methods. This paper reviews the relevant research in the frailty index based on the principle of cumulative defects and the approaches to its construction, common types and applications to provide new insights into and tools for frailty evaluation.