Abstract: OBJECTIVE To explore the pollution characteristics of breathing-circuit-systems in anesthesia machines and its influence on nosocomial infection. METHODS From Mar. 2015 to Dec. 2016, 150 anesthesia machines were randomly selected from the operating room of our hospital. After the operation was completed and the anesthesia machine was removed, 7 parts of breathing-circuit-systems were sampled (1,050 specimens), and the bacteria culture and identification were conducted in the laboratory. Bacterial contamination status of the sampling sites in the breathing-circuit-systems of anesthesia machines was recorded. Univariate and multivariate logistic analysis were performed on the factors that might cause the infection of breathing circuits in anesthesia machines. The influence of anesthetic machine pollution on the incidence of lower respiratory tract infection during postoperative hospitalization was analyzed. RESULTS Of the 150 anesthesia machines, 56 were contaminated, with a contamination rate of 37.33%, and among the 1,050 specimens collected, 188 specimens were positive in bacterial culture. Among all sampling sites, the positive rate of bacterial culture was the highest in the check valve of the exhalation port, accounting for 29.26% (55/188). Of the 188 bacterial culture positive specimens, 213 strains were isolated, including 113 strains of gram-negative bacteria (53.05%), 68 strains of gram-positive bacteria (31.92%) and 32 strains of fungi (15.02%). Multivariate logistic analysis showed that surgery time of > 3 hours, age of >60 years old and endoscopic surgery were the independent risk factors for the contamination of breathing-circuit-systems in the anesthetic machine (P<0.05). CONCLUSION Surgical time, patient age and endoscopic surgery were the risk factors for the contamination of breathing-circuit-systems in the anesthetic machine. These patients are more susceptible to lower respiratory infection during postoperative hospitalization.