The wide application of antibiotics in aquaculture requires an efficient treatment of the wastewater before discharging it into the environment. During the wastewater treatment, the influence of antibiotics on the performance of bioreactor should be well revealed due to their toxicity to the functional microbial community. In this study, the effect of feeding 10–30 mg L –1 sulfamethoxazole (SMX) in influent on the performance of an anoxic/oxic-moving bed biofilm reactor (A/O-MBBR) treating mariculture wastewater and the responding change of biofilm microbial communities was investigated. The COD average removal rate remained at 94.61%–97.34% with the dosage of SMX. Compared with that, the nitrifying removals of NH 4 +-N and NO 2 –-N were violently inhibited by 30 mg L –1 SMX and denitrifying removal of the NO 3 –-N decreased obviously with 20 mg L –1 or more SMX. The microbial community in the successful startup bioreactor was relatively abundant, while the diversity of microbial community decreased with the increase of feeding SMX. The salt-tolerant and SMX-resistant genera Arcobacter, Thiothrix, Desulfuromusa and Nitrosomonas were gradually enriched and finally played a vital role in converting COD and recycling nitrogen and sulfur. Hence, the present A/O-MBBR reactor with the salt-tolerant functional microbiota achieved efficient removal of pollutants in the presence of low concentration ( e.g., 10 mg L –1) SMX.