Abstract: Subsyndromal Delirium (SSD) patients often show symptoms such as fluctuation of consciousness level, inattention, perceptual and thinking disorders. They don't meet the diagnostic criteria of full delirium, but their clinical outcomes will be impacted by SSD severely. SSD will lead to higher mortality, prolonged length of hospital stay and duration of mechanical ventilation.SSD has been highly concerned by researchersin the world. This article reviewed the epidemiology, etiology, diagnosis, treatment and clinical outcomes of SSD, so that to provide evidence for nurses to recognize and monitor patients' consciousness level in an effective way.