The glyceraldehyde conversion in terms of dehydration, keto-enol tautomerism, and retro-aldol reaction has been discussed by means of ab initio density functional theory calculation to elucidate the catalytic water molecule influence on the pathways. The optimized structures of reactants, products, and transition state structure on the glyceraldehyde conversion pathways were fully optimized with the B3LYP/AUG-cc-pVDZ level. At first, the retro-aldol reaction through a six-membered ring-like transition state structure reduced the threshold energy, which was preferred pathway in the absence of water. The presence of water on the dehydration and the tautomerism pathways reduces the threshold energies, but dehydration is consistently more favorable than tautomerism and retro-aldol reaction. Water molecule can enhance glucose conversion by directly participating in the transition state by forming ring-like water bridge and lowering the threshold energy. In the future study, the discrete solvation model simulation in terms of the solvation effect will be investigated on the glyceraldehyde conversion in supercritical water by means of quantum mechanics / molecular mechanics (QM/MM) simulation.