The brackish tidal marsh in the Baimaosha area of the Yangtze River Estuary was severely contaminated by 400 tons of heavy crude petroleum from a tanker that sank in Decem-ber 2012.The spill accident led to severe environmental damage owing to its high toxic-ity,persistence and wide distribution.Microbial communities play vital roles in petroleum degradation in marsh sediments.Therefore,taxonomic analysis,high-throughput sequenc-ing and 16S rRNA functional prediction were used to analyze the structure and function of microbial communities among uncontaminated(CK),lightly polluted(LP),heavily polluted(HP),and treated(TD)sediments.The bacterial communities responded with increased rich-ness and decreased diversity when exposed to petroleum contamination.The dominant class changed from Deltaproteobacteria to Gammaproteobacteria after petroleum contam-ination.The phylum Firmicutes increased dramatically in oil-enriched sediment by 75.78％,346.19％and 267.26％in LP,HP and TD,respectively.One of the suspected oil-degrading gen-era,Dechloromonas,increased the most in oil-contaminated sediment,by 540.54％,711.27％and 656.78％in LP,HP and TD,respectively.Spore protease,quinate dehydrogenase(quinone)and glutathione-independent formaldehyde dehydrogenase,three types of identified en-zymes,increased enormously with the increasing petroleum concentration.In conclusion,petroleum contamination altered the community composition and microorganism struc-ture,and promoted some bacteria to produce the corresponding degrading enzymes.Addi-tionally,the suspected petroleum-degrading genera should be considered when restoring oil-contaminated sediment.