During the laser activity window ( from 6.60 fs to 6.84 fs), the results with the uniform field and the near field are quite dissimilar. In the uniform field, the force vectors in the XZ plane act on the H 2 O molecule are affected by the field and electron injection, and its direction and magnitude change rapidly. While in the near field, the force vectors in the XZ plane always point to the 'tips' of Au 20 , especially the force acting on O atom. It is known that this direction is also the negative gradient direction of the field. We thus suggest that the inhomogeneous near field affects the water splitting reaction through the gradient force and causes a different reaction during the field active window.