Directly splitting water into carbon-free H ₂fuel and O ₂gases by sunlight is one of the most environmentally-friendly and potentially low cost approaches to solving the grand global energy challenge. Recent progress of electronic structure theory and quantum simulations allow us to directly explore the atomistic mechanism and ultrafast dynamics of water photosplitting on plasmonic nanoparticles. Here in this paper, we briefly introduce the relevant researches in our group. First we propose that the supported gold nanoparticles on oxide thin film/mental should be able to potentially serve as efficient photocatalysts for water splitting. Then, under the light illumination, we identify a strong correlation among light intensity, hot electron transfer rate, and water splitting reaction rate. The rate of water splitting is dependent not only on respective optical absorption strength, but also on the quantum oscillation mode of plasmonic excitation, which can help to design nanoparticles in water photosplitting cells. Finally, we simulate the ultrafast electron-nuclear quantum dynamics of H ₂generation with plasmonic gold cluster on a time scale of~100 fs in liquid water. We identify that the water splitting is dominated by field enhancement effect and associated with charge transfer from gold to antibonding orbital of water molecule. Based on all atomistic mechanism and quantum dynamics above, we present a “chain-reaction” H ₂production mechanism via high-speed (much higher than their thermal velocity) collision of two hydrogen atoms from different water molecules under light illumination.