Polymyxin B(PMB)is considered as the last line of antibiotic defense available to humans.The environmental effects of the combined pollution with PMB and heavy metals and their interaction mechanisms are unclear.We explored the effects of the combined pollution with PMB and arsenic(As)on the microbial composition of the soil and in the earthworm gut,as well as the spread and transmission of antibiotic resistance genes(ARGs).The results showed that,compared with As alone,the combined addition of PMB and As could signifi-cantly increase the bioaccumulation factor and toxicity of As in earthworm tissues by 12.1％and 16.0％,respectively.PMB treatment could significantly increase the abundance of Acti-nobacteria in the earthworm gut(from 35.6％to 45.2％),and As stress could significantly increase the abundance of Proteobacteria(from 19.8％to 56.9％).PMB and As stress both could significantly increase the abundance of ARGs and mobile genetic elements(MGEs),which were positively correlated,indicating that ARGs might be horizontally transferred.The inactivation of antibiotics was the main resistance mechanism that microbes use to resist PMB and As stress.Network analysis showed that PMB and As might have antagonis-tic effects through competition with multi-drug resistant ARGs.The combined pollution by PMB and As significantly promoted the relative abundance of microbes carrying multi-drug resistant ARGs and MGEs,thereby increasing the risk of transmission of ARGs.This research advances the understanding of the interaction mechanism between antibiotics and heavy metals and provides new theoretical guidance for the environmental risk assessment and combined pollution management.