Based on ab-initio principle and pseudo-potential plain wave function combined with density function theory, crystal lattice dynamical characteristics and thermodynamical functions of V, VH2, VD2 and VT2 were calculated. The thermodynamical isotope effect was analyzed based on those calculated data. Calculation results show that both difference of vibration frequency of atoms in VQ2 (Q=H, D and T) and difference of oscillation frequency of hydrogen isotopes are mainly responsible for the hydrogen isotope effect. The standard Gibbs free energy change of the absorption reaction rises from –50 kJ/mol to 140 kJ/mol with the temperature increasing from 10 K to 1000 K. The equilibrium pressure climbs drastically when temperature is higher than room temperature. Within the whole temperature range, only inverse hydrogen isotope effect is found