Currently,　sulfadiazine　(SDZ)　and　polyvinyl　chloride　(PVC)　microplastics　were　commonly　detected　in　wastewater,　causing　widespread　concern.　However,　little　is　known　about　the　fate　of　SDZ　in　an　anaerobic/anoxic/oxic　(AAO)　system　and　the　response　of　activated　sludge　and　PVC　biofilms　under　SDZ　stress.　Thus,　this　study　explored　the　effects　of　5　mg/L　SDZ　on　the　biological　activities,　microbial　communities　and　the　propagation　of　resistance　genes　(RGs)　of　activated　sludge　and　PVC　biofilms　during　140　days　in　an　18　L　AAO　system.　The　nitrogen　and　phosphorus　removal　efficiency　decreased　from　90.67%　and　93.04%　to　29.01%　and　25.17%　at　the　6th　day　under　SDZ　stress,　respectively,　and　then　gradually　increased　with　the　degradation　of　SDZ.　Additionally,　there　were　significant　discrepancies　in　metabolic　capacities　and　microbial　community　structures　among　PVC　biofilms　collected　from　different　zones　of　AAO　and　activated　sludge.　Potential　pathogens　Acinetobacter　lwoffii　and　Mycolicibacterium　mageritense　only　colonized　on　PVC　surfaces.　With　the　loading　of　SDZ　during　140　days,　the　abundance　of　sul1　and　intI1　continuously　increased　in　PVC　biofilms.　And　sul1　was　the　highest　RGs　and　might　transfer　from　Pseudomonas　to　Thauera　in　activated　sludge,　and　some　RGs　were　enriched　and　co-located　with　mobile　genetic　elements　in　hosts,　causing　higher　dissemination　risks　of　RGs.　The　findings　of　this　study　might　provide　new　insights　into　the　biological　response　discrepancies　of　activated　sludge　and　PVC　biofilms　under　SDZ　stress.