Four lowest Ω substates (X2Π3/2,g, X2Π1/2,g, A2Π3/2,u and A2Π1/2,u) of the I+2 cation are studied by high-precision ab?initio calculations, and the calculated results are compared with the experimental high-resolution spectra of literatures. Firstly, the potential energy curves of the four Ω substates are calculated using the multi-reference configuration interaction (MRCI) method, and the rovibrational levels of these electronic states are derived by solving the radial Schrodinger rovibrational equation. Then, molecular constants are obtained by fitting energy levels to a spectroscopic model, and based on the fitted spectroscopic constants and calculated transition dipole moment matrix elements, line strengths of vibronic bands in the A2Π3/2,u- X2Π3/2,g system for 45 bands with υ'=11-19 and υ''=1-5 have been derived, and the Einstein A coefficients are employed to compute radiative lifetimes of the υ'=11-19 vibrational levels of the A2Π3/2,u state. Finally, based on the calculated predissociation lifetimes of the higher vibrational levels, the predissociation mechanism of the A2Π1/2,u state is discussed.