Based on the potential parameter Ω, which was defined as energy deposits ratio of incidence ion with target ion after double electron capture, the selection rule between double electron capture and transfer ionization of helium target induced by Aq+ (q=4,5,6,7) ions was studied. It is found that there is a cross point for the cross-section ratio curve of double electron capture and transfer ionization at Ω=10, and for ΩΩ≥10 the transfer ionization is a key reaction channel of double electron transfer.