Objective To investigate the contamination status of dental unit waterlines (DUWLs), and explore influencing factors for DUWLs contamination. Methods The stratified random sampling method was used to collect water specimens from different brands of dental chair unit (DCU) in various outpatient departments of a stomatologi-cal hospital in Beijing from June 22, 2017 to February 23, 2018, bacterial culture and counting was performed, univariate and multivariate analysis of variance was adopted to analyze the differences in bacterial colony of water specimens from DUWLs of different brands, service years, used clinics and sampling sites. Results A total of 464 DCU water specimens were collected, the median of colony forming unit of DUWLs water specimens was 2.6×10³(17-5.2×10⁵)CFU/mL. Univariate analysis of variance showed that there were significant differences in colony forming unit of water specimens from DUWLs of different service years, different diagnosis and treatment specialties and different brands (all P<0.05), but there was no significant difference in colony forming unit of water specimens at different outlet of DCU (P=0.067). Multivariate analysis of variance showed that service years and brands of DCU, as well as diagnosis and treatment specialties all influenced the change of DUWLs bacterial colony (P<0.001), and there was an interaction between DCU brands and diagnosis and treatment specialties, DCU brands and service years (both P<0.001). Conclusion At present, DUWLs are seriously contaminated and are jointly affected by various factors such as DCU service years, brands, as well as diagnosis and treatment specialties.