Vitamin D is an important fat-soluble vitamin for maintaining vital activities. It has a variety of biological effects and pathophysiological significance. A growing number of studies are focusing on predictive value of vitamin D level and therapeutic effects in critically ill patients. Due to reduced intake, metabolic changes, increased clearance and blood dilution of vitamin D, vitamin D insufficiency or deficiency is prevalent in critically ill patients. It was proved that insufficient vitamin D was associated with poor outcomes in critically ill patients. Reviewing different studies exploring the therapeutic effects of vitamin D supplementation in critically ill patients, there were great heterogeneities in recruited population, research methods and results. Whether vitamin D supplementation helps improve clinical outcomes in critically ill patients is controversial. The target adaptation population of vitamin D treatment, the method and standard of monitoring and evaluation, the preparation and dosage, and the timing of supplementation may all affect the efficacy evaluation of vitamin D. It is of great significance to strengthen the monitoring of vitamin D level and to explore the effect of vitamin D supplementation on clinical outcomes in critically ill patients in China. This paper mainly discusses the application value and application prospect of vitamin D in the treatment of severe diseases.