Abstract: As the largest microbial community in the human body, the intestinal microecology plays a pivotal role in the health and disease progression of the host. As part of the human symbiotic system, the metabolites of the intestinal microecology have a profound impact on the physiological and pathological states of the host. Hospital-acquired infections (HAIs), as a common health problem among hospitalized patients, involve complex nechanisms influenced by multiple factors. In recent years, with the advancement of technologies such as 16S rRNA and shotgun sequencing, researchers have gradually recognized that the intestinal microecology plays an important role in the occurrence and development of HAIs. In the intensive care unit, the treatment of primary diseases, secondary infections, and the use of clinical treatment methods often lead to intestinal microecological imbalance. This article reviews the characteristics of changes in the intestinal microecology of critically ill patients after admission and explores the relationship between these changes and HAIs, aiming to deeply explore the possible mechanisms of the intestinal microorganisms in affecting the hospital-associated infections and provide theoretical support for developing prevention and treatment strategies.