In this article,the protonation processes before and after decarboxylation of oro tidine 5 -monophosphate(OMP)catalyzed by orotidine 5-monophosphate decarboxylase(ODCase)is investigated theoretically at both Hartree-Fork and density functional theory(DFT)levels.The results indicate that firstly O4 is protonated by residues of ODCase bridged by water molecules.Then,after decarboxylation,a fast stepwise protonation process happens.Additionally,the catalytic function of Lys-93 is also discussed.The protonation process plays an important role in the whole reaction.