The work aims to study the metal alloying of RuAl and the oxidation relation between Al2O3 and RuO2 to reveal the anti-oxidation mechanism. Based on the first principle of density functional theory, the RuAl-X-O cell model of RuAl doped metal atom X and its interstitial O atom and the cell model of its oxidation products Al2O3-X and RuO2-X were established. The calculated oxidation energy of Al2O3 and RuO2 showed that oxidation energy of Al2O3 (-11.43 eV/O2) was smaller than that of RuO2 (-2.28 eV/O2) and the oxidation energy of RuO2 was close to 0. Under high temperature conditions, the structure had poor stability and was easy to decompose and the anti-oxidation of RuAl was mainly dependent on Al2O3. After metal X alloying, the oxidation energy of Al2O3-X and RuO2-X increased, and the difference of oxidation energy (eV/O2) from large to small was Zr(0.29)>Ce(0.28)>Sn(0.22)>Sr(-0.49), among which the effect of metal Zr alloying to improve the anti-oxidation of RuAl was the best. The calculated oxygen gap formation energy and charge density showed that the alloying of RuAl by metal X atom decreased the solid solubility of O in RuAl and the rate of internal oxidation in RuAl. The alloying of RuAl by metal X atoms hinders the diffusion of O on the surface of RuAl to its interior and the formation conditions of "internal oxidation". It is easy to form continuous and dense Al2O3 oxide layer in the transverse direction of RuAl surface interface to improve the oxidation resistance.