Microorganisms inhabited in oil reservoirs play a fundamental role in microbial enhanced oil recovery. Gas injection has been adopted as an alternative method in the tertiary oil recovery, but the influence of the gas injection on microbial composition in oil reservoir and its contribution to oil recovery still remained unclear. In this study, the effect of air injection on the microbial composition and transcriptional activity is investigated using a combination of the 16S rRNA gene amplicon sequencing, metagenomic and meta-transcriptomic analysis. Firmicutes were the main active bacteria in all samples and the proportion of Firmicutes increased with the rising of gas/liquid ratio in samples without additional treatment. The Bacillus was the dominant member of phylum Firmicutes. With the increase of gas-to-liquid ratios, the microbes showed higher overall transcription activities and propionate become an important metabolite. This research provides theoretical guidance for the activation of endogenous oil displacement microorganisms and sheds light on the industrial applications for exploitation of depleted reservoirs and the extension of the development life of oil reservoirs.