OBJECTIVE： To investigate the effects of sub-MIC antibiotics on the biofilm formation of multi-drug resistant Acinetobacter baumannii （MDR-AB）， and to provide reference for the prevention and treatment of related infection. METHODS： Agar plate doubling dilution method was used to detect MIC of clinically isolated A. baumannii （AB） and screen MDR-AB. Microporous method was used to analyze the effects of different doses of sub-MIC antibiotics on the biofilm formation of MDR-AB， which was validated by using AB standard strain （ATCC 17978）. The expression of biofilm-related gene was detected by RT-PCR. RESULTS： The detected MDR-AB was resistant to carbapenems， cephalosporins， quinolones， aminoglycosides， tetracyclines and macrolides （MIC≥16 μg/mL）. It was sensitive to polymyxin B and tegocycline （MIC≤8 μg/mL）. Different doses of sub-MIC cefepime， ciprofloxacin， azithromycin and amikacin could significantly inhibit the biofilm formation of MDR-AB， and azithromycin had the strongest inhibitory effect （the relative biofilm formation values of the corresponding strains was minimal）. Doxycycline could significantly induce the formation of biofilm in most strains， but meropenem， cefoperazone， tigacycline， gentamicin and polymyxin B had no significant effect on the formation of biofilm. Results of validation test showed that compared with untreated control strains， the relative biofilm formation values of standard strains were decreased significantly after treated with 0.25 and 0.125 μg/mL azithromycin and 0.25 μg/mL amikacin （P＜0.05）， and inhibitory effect of azithromycin showed a dose-dependent relationship （P＜0.05）. Results of gene expression detection showed that compared with untreated control strains， relative expression of bap， filA， pbp-1a and pbp-1b gene were decreased significantly （P＜0.05）， while relative expression of ompA and csuE gene had no significant change （P＞0.05）. CONCLUSIONS： The drug resistance of MDR-AB strains in our hospital is serious. Sub-MIC cefepime， ciprofloxacin， amikacin and azithromycin shows significant inhibitory effect on biofilm formation， and the inhibitory effect of azithromycin is the strongest， with dose-dependent manner. Azithromycin can inhibit the biofilm formation of MDR-AB which may be associated with the expression down-regulation of bap， filA， pbp-1a and pbp-1b gene.