Developing a new ultrasensitive interface to detect As(III) is highly desirable because of its seriously toxic and low concentration in drinking water. Recently, Fe3O4 nanoparticles of high adsorption toward As(III) become very promising to be such an interface, which is still limited by the poor understanding of their surface physicochemical properties. Herein, we report that dumbbell-like Au/Fe3O4 nanoparticles, when being modified the screen-printed carbon electrode, can serve as an efficient sensing interface for As(III) detection with an excellent sensitivity of 9.43 μA ppb-1 and a low detection limit of 0.0215 ppb. These outstanding records were attributed to the participation of Fe(II)/Fe(III) cycle on Fe3O4 surface in the electrochemical reaction of As(III) redox, as revealed by X-ray photoelectron spectroscopy, X-ray absorption near edge structure, and extended X-ray absorption fine structure. This work provides new insight into the mechanism of electroanalysis from the viewpoint of surface active atoms, and also helps to predict the construction of ultrahighly sensitive electrochemical sensors for other heavy metal ions with nonprecious redox active materials.