In this work, we investigate the dynamic process of water splitting driven by Au cluster and NPs with a few nanometers in size under femtosecond laser pulse. We explore the related mechanisms of H 2 O splitting and hot electron injection, and reveal the relationships between the reaction rate and the incident light intensity and frequency, and NP's sizes. The real-space real-time TDDFT/Ehrenfest dynamics schemes is adopted. Four model systems, H 2 O and Au 20 @H 2 O interacting with the laser field and the near field generated by different sized NPs, have been investigated. The water molecule is assumed to bind with one of the vertexes of Au 20 . Au 20 has a tetrahedral structure and is highly stable and chemically inert. The tetrahedral Au20 is a fragment of the face-centered cubic lattice of bulk gold with a small structural relaxation, a unique molecule with atomic packing similar to that of bulk gold. For H 2 O and Au 20 @H 2 O systems, their electronic degrees of freedom were treated quantum mechanically within the real-space real-time implementation of time-dependent density functional theory (TDDFT), while their nuclei were handled classically.