Pt modified CoNiCrAl medium-entropy alloys (MEAs) were optimized that displayed outstanding long-term oxidation resistance life at 1100 °C. Compared to air, introducing interstitial H into the Al2O3 lattice changed the energy of Al vacancy formation, from 6.89 eV to 4.13 eV, indicating that water vapour accelerated the oxidation of MEAs, which was twice that of air. With the Pt increasing, the Young's modulus and hardness improved, promoting the formation of smoother Al2O3 scale in 10Pt-MEA. Precipitation of Pt enrichment at the phase boundary suppressed oxygen diffusion and reduced oxidation rate. The designed MEAs expand the application of high/medium-entropy alloys. •Pt modified medium-entropy alloys were designed and optimized.•Introducing H into the Al2O3 lattice reduced the formation energy of Al vacancy.•Precipitation of Pt enrichment at the phase boundary reduced the oxidation rate.•Pt solid solution in BCC phase improved the oxidation resistance.•A smoother Al2O3 scale formed in 10Pt-MEA.