Because of aging, multimorbidity and comobidities, the organ function reserve declines in elderly critically ill patients who are predisposed to infection, malnutrition, adverse drug reaction and organ injury. It results in very complex situation. Elderly critically ill patients suffer more complications, high incidence of multiple organ dysfunction syndrome, and increased mortality. Clinicians should attach importance to the disease complexity for elderly critically ill patients and take effective measures to improve their coutcome.