To study the chemical composition,antioxidant and antimicrobial activities of Elsholtzia ciliata essential oil (ECEO).ECEO was extracted by steam distillation method,and the chemical compositions were analyzed by gas chromatography-mass spectrometry (GC-MS) and fourier transform infrared spectroscopy (FTIR).The antioxidant ability of ECEO was evaluated by DPPH and β-carotene bleaching methods. The minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) value of ECEO was determined,the effect of ECEO on the pneumonia model infected by P. aeruginosa was studied.The results showed that the main components of ECEO were thymol (79.17%) and 3-methyl-4-isopropylphenol (12.35%),the IC₅₀ of DPPH scavenging ability and β-carotene oxidation inhibition ability were 2.05 and 6.93 μg/mL,respectively.The essential oil showed obvious inhibitory effect on both sensitive and resistant bacteria,and it could also had an effective remission effect on pneumonia induced by P. aeruginosa infection,in addition,ECEO could prevent or postpone the development of pneumonia in mice caused by levofloxacin resistant P. aeruginosa superinfection.The results suggested that ECEO had superior antioxidant activity and broad-spectrum antibacterial effects.