Abstract: The model of H_2O-H_2 isotopes catalytic exchange reaction was described and proved in terms of dynamics and the process of reaction. The results are shown as following: three-phase (gas/vapour, liquid, solid) coexist in the system of H_2O-H_2 isotopes catalytic exchange reaction. It is a very complex tandem process of the mass transfer and reaction, which mainly involves two steps, one is phase transform between vapour and liquid water in hydrophilic packing; the other is catalytic exchange of hydrogen isotopes in hydrophobic catalyst.