The mechanisms of the aromatic Claisen rearrangement of 1-(4-chloronaphthyl) 1,1-dimethylallyl ether (Re) under neat conditions and "on water" were investigated. The aromatic Claisen rearrangement usually involves the [3,3]-intramolecular shift followed by a proton transfer. The intermolecular proton transfer is the rate-limiting step under neat conditions with Delta Delta E-b and Delta Delta G(double dagger). values of 25.7 and 29.8 kcal/mol at the B3LYP/6-311++G(d,p) level, respectively. The on water condition was simply modeled by a combination of the "oil" droplet/water interface and neat condition inside the oil droplet. The MD simulation was used to obtain the most reliable interaction position between Re and solvent water, which was further used as a starting material for the water-catalyst mechanism to model the surface reaction. We found that the chairlike [3,3]-intramolecular shift became the rate-limiting step for the water-catalyst mechanism, with lower Delta Delta E-b (16.3 kcal/mol) and Delta Delta G(double dagger) (25.2 kcal/mol) values compared with those under neat condition. Their Delta Delta E-b and Delta Delta G(double dagger) values changed to be 22.0 and 24.9 kcal/mol, respectively, after considering the bulk water effect by QM/MM calculation. Hence, these calculated energy results strongly suggested that the on water reaction should be faster than the one under neat conditions. This can be explained by the following three key factors: (1) the interaction between the species and water clusters in the transition states, especially for the proton transfer process, is stronger than in other states, which was revealed by the binding energy calculation; (2) the two-water cluster enhanced the charge separation in the reaction center of the [3,3]-intramolecular shift, increasing the stability of the corresponding transition state; and (3) the donor-acceptor NBO results suggested that the hydrogen-bonded two-water cluster accelerated the proton transfer process by serving as a proton bridge.