To investigate the tautomerism of uracil−(water) n (n = 0−4) that is induced by proton transfer, we describe a study of structural tautomer interconversion and the relative stabilizing influences of water on uracil (U) and its enol form (U*), using density functional theory (DFT) calculations by means of the B3LYP exchange and correlation functions. Thirty-three geometries, including seven significant transition states, were optimized, and their geometrical parameters have been discussed in detail. Water molecules have been gradually placed in different regions in the vicinity of U and U*, and the intermolecular interactions between U, U*, and water molecules have been studied. The relative stabilities of all tautomers were established. The calculated results indicate that there are two absolutely opposite regions in the vicinity of uracil. In one of the regions, water molecules can protect U from tautomerizing to U*, whereas in another region, water molecules can assist the tautomerism from U to U*. The stabilization and mutagenicity effects of increasing the number of water molecules in only one region and in both regions have been systematically studied and discussed.