We investigated the application of liquid chromatography-tandem mass spectrometry (LC-MS/MS) for pan-drug-resistant Acinetobacter baumannii (PDR-AB) with carbapenem resistance. Eight strains were randomly selected from 84 clinical isolates of PDR-AB strains obtained by the Kirby-Bauer and agar dilution methods. An efflux pump inhibition test was used to screen for the efflux pump phenotype. An ethylenediaminetetraacetic acid (EDTA) synergy test was used to screen for the β-lactamase phenotype, and a three-dimensional test was used to detect extended spectrum β-lactamase (ESBL) and ampicillin C, KPC, and carbapenemase. ESBL genes were amplified by polymerase chain reaction and sequenced. Outer membrane proteins were extracted from a sensitive strain and the PDR-AB strains by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and subjected to LC-MS/MS. Peptide mass fingerprinting data were retrieved, and proteins with differential expression were identified.