<p>Metallic combustion in oxygen-enriched environments remains one of the most unpredictable failure modes in aerospace propulsion and life-support systems, but design rules linking alloy chemistry to burning resistance are still missing. Exploiting the structurally simple, single-phase solid solution of CoCrFeNi high-entropy alloys (HEAs), we systematically investigated effects of Cr on flame retardancy with multiscale characterization and thermodynamic modeling. A sharp optimum at 30 at.% Cr raises the sustained-burning threshold by 89% (1.8 → 3.4 MPa), exceeding all conventional superalloys. Based on electron probe microanalysis (EPMA) element distribution and finite element thermal simulations, it is concluded that a liquid Cr₂O₃ film forms on the melt pool during combustion. This film exhibits self-healing behavior and effectively suppresses oxygen ingress, representing a barrier mechanism distinct from traditional solid oxide scales. A valence-electron-concentration (VEC) criterion of ⩽8.0 quantitatively predicts the FCC→BCC transition that triggers sudden loss of flame resistance, converting electronic structure into an immediately applicable safety boundary. CoCrFeNi series HEAs exhibit a peak yield stress over 720 MPa without external coatings when deformed at 760 °C, providing a ready-to-use intrinsically flame-proof structural material. This work redefines Cr as a combustion-rate tuner rather than a simple oxide former and offers a universal design framework for next-generation combustion-proof high-performance alloys.</p>