ObjectiveTo evaluate the clinical efficacy and prognosis of cefoperazone/sulbactam combined with thymosin α1 in the treatment of severe pneumonia caused by  Acinetobacter baumannii(A. baumannii ). Methods84 patients with severe pneumonia caused by  A. baumannii  were randomly selected, they were divided into treatment group(n=42, cefoperazone/sulbactam combined with thymosin α1 treatment) and control group(n=42, only cefoperazone/ sulbactam treatment). Procalcitonin（PCT）, Creactive protein（CRP）, white blood cell（WBC）count, peripheral blood T lymphocyte subsets, interleukin6（IL6）, interleukin10（IL10）, immunoglobulin G (IgG), and APACHE II score of two groups before treatment and 7 days after treatment were compared, ventilator weaning success rate, length of ICU stay, and 28day mortality were also observed. ResultsAfter 7 day treatment, compared with the control group, CD4+T cells, CD4+／CD8+, IL10, and IgG in the treatment group were all significantly higher (all P<0.05); PCT, CRP, WBC, IL6, and APACHE II score all significantly declined, difference were all significant(all P<0.05). Ventilator weaning success rate in treatment group was higher than control group（64.29% vs 38.10%），mean length of ICU stay was shorter than control group（［12.41±2.25］d vs［18.23±2.50］d），28day mortality was lower than control group（19.05% vs 45.24%）, difference were all significant(all P<0.05). ConclusionCefoperazone/sulbactam combined with thymosin α1 for the treatment of severe pneumonia caused by  A. baumannii  can improve the immune function of patients, reduce inflammation, increase ventilator weaning success rate, shorten ICU stay， and decrease 28day mortality.