Realizing efficient and selective separation of monovalent and divalent cations in water is the key to the development of next-generation water treatment and resource recovery technologies. Herein, we report a simple slight oxidation strategy that results in selective oxidation of Ti atoms and retention of V atoms on the surface of the double transition-metal TiVCTx (abbreviation TiVC) MXene. The TiO2 nanoparticles partially derivatized on the surface due to slight oxidation extended the layer spacing of MXene and improved its ion diffusion kinetics, while providing abundant active sites. The retention of V atoms ensures the integrity and good electronic conductivity of the MXene 2D lamellae. Benefiting from the above synergistic effect, SO-TiVC (slightly oxidized TiVCTx MXene) possesses good electrochemical performance as well as hybridized CDI performance, showing high ion selectivity for divalent cations Ca2+ and Mg2+ (SCa/Na = 13.67, SMg/Na = 11.12), which is superior to that of the reported MXene-based electrodes. Density-functional theory calculations show that the expansion of the MXene layer spacing and the change in the surface end-groups combine to promote the higher divalent cation selectivity of SO-TiVC. This study provides a theoretical basis for the development of efficient divalent cation-selective electrodes utilizing the intrinsic properties of the bis-transition metal MXene for environmental separation applications.