It is important to study the solvent effect on keto-enol tautomerism that has applications in many areas of chemical engineering. In this work, we use a multiscale reaction density functional theory (RxDFT) to study the keto-enol tautomerism and isomerization of pyruvic acid. The results show that both effects of solvation and water assistance could reduce the reaction barriers. The water molecule participates the reaction as a catalyst to accept/give the protons with forming a hexagonal ring in the transition state. As a result of this temporary and intermediate hexagonal ring, the solute configuration undergoes a small variation during the reaction, giving a diminished contribution to the intrinsic reaction free energy. The solvent distribution shows a local ordering behavior near the solute that also reduces the contribution of solvation effect to the reaction barrier. Water assistance plays a major role in both pre-reaction and post-reaction process. In terms of the driving force for the reaction, the effects of both solvation and water assistance are important.