Abstract: The one-carbon unit transfer reaction catalysed by glycinamide ribonucleotide transformylase（GARTfase）is a key step in the de novo purine biosynthetic pathway. In order to give a theoretical research of the assumption from the experiments，the water-assisted mechanism in GAR Tfase catalysed one-carbon unit transfer reaction has been investigated by a Density Functional Theory method，B3LYP，at 6-31G* basis level. There are two possible reaction channels for the whole reaction，one is concerted（path a）and the other is stepwise（path b）. The water molecule serve as a bridge to connect the proton donor to the proton acceptor. All the transition states in both paths have a six-membered ring in their structures due to the joint of the water molecule. The calculations show that the latter is preferable to the former due to the lower energy barriers. The results have verified the presumption from experiments，and proved that the joint of a water molecule can relax the strong strain in the unstable system，so it is propitious to the whole reaction.