Conjunctive transfer of antibiotic resistance genes (ARGs) among bacteria driven by plasmids facilitated the evolution and spread of antibiotic resistance. Heavy metal exposure accelerated the plasmid-mediated conjunctive transfer of ARGs. Nanomaterials are well-known adsorbents for heavy metals removal, with the capability of combatting resistant bacteria/facilitating conjunctive transfer of ARGs. However, co-effect of heavy metals and nanomaterials on plasmid-mediated conjunctive transfer of ARGs was still unknown. In this study, we investigated the effect of the simultaneous exposure of Cd²⁺ and nano Fe₂O₃ on conjugative transfer of plasmid RP4 from Pseudomonas putida KT2442 to water microbial community. The permeability of bacterial cell membranes, antioxidant enzyme activities and conjugation gene expression were also investigated. The results suggested that the combination of Cd²⁺ and high concentration nano Fe₂O₃ (10 mg/L and 100 mg/L) significantly increased conjugative transfer frequencies of RP4 plasmid (p < 0.05). The most transconjugants were detected in the treatment of co-exposure to Cd²⁺ and nano Fe₂O₃, the majority of which were identified to be human pathogens. The mechanisms of the exacerbated conjugative transfer of ARGs were involved in the enhancement of cell membrane permeability, antioxidant enzyme activities, and mRNA expression levels of the conjugation genes by the co-effect of Cd²⁺ and nano Fe₂O₃. This study confirmed that the simultaneous exposure to Cd²⁺and nano Fe₂O₃ exerted a synergetic co-effect on plasmid-mediated conjunctive transfer of ARGs, emphasizing that the co-effect of nanomaterials and heavy metals should be prudently evaluated when combating antibiotic resistance.