Hot-electron injection is widely used in plasmonic devices. However, it is still lack of a direct theoretical model for performance prediction. This paper measures the surface potential of Au/TiO2 film by Kelvin probe force microscope (KPFM) under various conditions, and then develops a theoretical model for quantitative interpretation. The model can well fit the relationship of surface potential versus light power under various irradiation wavelengths. The study in this paper opens the pathway for quantitative characterization of the efficiency of hot-electron injection and sheds light on improving the plasmonic efficiency of photoelectric conversion and photocatalysis.