Sc2CO2 MXene has potential applications in energy storage and optoelectronics due to its superior structure and excellent properties. The electronic properties, quantum capacitance, and photocatalytic activity of Cr-doped Sc2CO2 under strain are studied by the density functional theory. Cr doping makes the system produce magnetism. The spin-down states of Sc2CO2-Cr under strain are direct semiconductors, while their spin-up states are indirect semiconductors. Sc2CO2-Cr under +5, -5, -3, and -2% strains in an aqueous system are suitable for cathode material. A large voltage drastically modulates the type of electrode materials. Sc2CO2-Cr under strains from 0 to +2% can perform the oxygen evolution reaction at an alkaline environment, while the Sc2CO2-Cr system under strain is a good for CO2 photocatalysis at pH 0 and 7.