Oilfield wastewater contains a large number of metal ions (such as Ca2+ and Cu2+). Reinjection in oil wells without treatment would affect oil recovery. To remove Ca2+ and Cu2+ ions, we prepared a reusable Fe3O4/GO-COOH nanoadsorbent by the magnetization and carboxylation of graphene oxide (GO). The Ca2+ and Cu2+ removal percents of the nanoadsorbent reached 78.4% and 51% at 60 min, respectively. After five adsorption/desorption cycles, the nanoadsorbent retained high recovery rates (82.1% for Ca2+ and 91.8% for Cu2+) and removal percents (72.3% for Ca2+ and 49.33% for Cu2+). In the simulated indoor oil recovery test, oilfield wastewater treated with Fe3O4/GO-COOH could enhance oil recovery (EOR) by 11.0% compared with untreated oilfield wastewater. The novel magnetic nanoadsorbent could not only adsorb metal ions in oilfield wastewater effectively to enhance oil recovery (EOR) but could also be quickly recycled and reused, which shows promising application prospects in oil exploitation. (C) 2021 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.