Figure shows the time evolution of the OH bond length in Au 20 @H 2 O under the intensive near field which generated by the truncated Au tetrahedrons with different sizes. Here the laser field with E 0 max = 0.50 V/Å and ω 0 = 3.07 eV is set. We observe that one of OH bonds breaks at 25.12 fs under the field enhancement by the truncated Au tetrahedron with L = 2.88 nm. In this case, the disassociation reaction takes place at E 0 max < 1.80 V/Å. Furthermore, we observe that the larger is the NP's size, the faster is the disassociation reaction. For example, at L = 4.32 nm, the two OH bonds's dissociation times are 21.47 fs and 27.80 fs while at L = 5.76 nm, they are 21.03 fs and 25.88 fs, respectively. For comparison, we also calculate the splitting dynamics of H 2 O molecule which interacts with the near field generated by the whole Au tetrahedron of L = 2.88 nm obtained by FDTD. In this case, no electron is injected to H 2 O, and only the field enhancement effect of Au NPs gets involved. As Figure shows, OH Bond length( Å) bond oscillates at a certain frequency, and there is no sign of splitting. These numerical simulations suggest that the near field can enhance the reaction rate of water splitting and one can adjust the size of metal NP to control the plasmon-assisted photochemistry via field enhancement effect. However, without the injection of hot electrons, the water splitting reaction will not take place, which highlights the decisive factor to drive the water splitting.