An atomic-level Au nanocluster, as an excellent photocatalyst, is generally not considered as an efficient electrocatalyst due to its poor stability. Herein, a method is proposed to stabilize abundant Au25 on Fe2O3 nanoplates (Au25/OV-Fe2O3) successfully with oxygen vacancies (OV) created. Au25/OV-Fe2O3 shows superhigh catalysis in the electrochemical reduction toward As(III). The record-breaking sensitivity (161.42 μA ppb-1) is two orders of magnitude higher than currently reported, where an ultratrace limit of detection (9 ppt) is obtained, suggesting promising applications in the analysis of organic and bioactive substances. The stability of Au25 is attributed to the Au-Fe bond formed after loading Au25 nanoclusters on Fe2O3 nanoplates through "electron compensation" and bond length (Au-S) shortening. Moreover, the ligand S atoms in Au25 nanoclusters significantly contribute to the reduction of As(III). The fantastic stability and superior catalytic ability of Au25/OV-Fe2O3 provide guidelines to stabilize Au nanoclusters on metal oxides, indicating their potential electroanalytical applications.