The invention discloses a novel graphene oxide adsorbent TGO functionalized by 5-methylbenzotriazole and provides a synthetic method of the novel graphene oxide adsorbent, capable of realizing selective coadsorption of lanthanide/actinide elements in radioactive wastewater, belonging to the technical fields of treatment of radioactive wastewater in nuclear wastes and environmental protection. Theadsorbent TGO is only synthesized in two steps, the ligand grafting rate is 6.4%, and the adsorbent is simple in process, environment-friendly, high in reproducibility and excellent in product stability. The TGO has high adsorption capacity and excellent selective coadsorption property on the lanthanide/actinide elements under the involved adsorption conditions, the selectivity coefficient reaches100%, and selective separation between lanthanide/actinide metal ions and monovalent and divalent metal ions can be realized. The TGO is capable of easily performing solid-liquid separation after metal ions are adsorbed, and can be repeatedly utilized. An effective method is provided for co-extraction of the lanthanide/actinide elements in low and intermediate level radioactive waste liquor, anda novel thought is provided for realizing lanthanide/actinide separation.