The oxidation behavior of AlCoCrFeNiCu_0.5 high-entropy alloy (HEA) in air at both 800 °C and 900 °C has been analyzed and corresponding microstructural evolution after oxidation has also been studied. The experimental studies revealed that the as-cast AlCoCrFeNiCu_0.5 alloy is mainly composed of BCC and FCC phases, moreover, there is a spinodal structure with the constituent phases being NiAl-B2 and FeCr-BCC. After annealing at 900 °C for 6 h, the spinodal structure disappeared, but there was σ phase at grain boundary. Oxidation caused significant changes in the matrix of the alloy. The primary phases of the oxidized matrix are the NiAl-B2 phase and FeCoCr-FCC1 phase. A substantial amount of Al was consumed in matrix to form Al₂O₃ on the surface, resulting in the formation of Al-depleted layer. The longer the oxidation time, the thicker the Al-depleted layer, and a concentrated distribution of Cu-rich FCC2 phase was observed in this region. The isothermal oxidation kinetics of the alloy at both 800 °C and 900 °C followed the parabolic law. The k_p values for oxidation at 800 °C and 900 °C were 7.367 × 10^− 14 (g²·cm^− 4·s^− 1) and 2.105 × 10^− 13 (g²·cm^− 4·s^− 1) respectively, indicating that the kp value at 900 °C was 2.86 times that at 800 °C. A continuous Al₂O₃ layer on the surface being the key to its superior oxidation resistance of the HEA at both temperatures.