Fe-based amorphous alloys are widely used in electronic devices such as high-frequency transformers and choke cores due to their low coercivity, low loss, and high saturation magnetic induction intensity. However, these alloys have a relatively low crystallization temperature and are prone to oxidation, which limits their application in high-temperature environments. The addition of copper and niobium elements can suppress the growth of crystal nuclei and improve thermal stability. However, the impact on the alloy's high-temperature oxidation resistance and structural evolution remains unclear. This paper uses static air oxidation to investigate the microstructure evolution of Fe_73.5Si_13.5B₉Cu₁Nb₃ amorphous alloy after high-temperature oxidation and its impact on magnetic properties. Besides, long-time oxidation, such as 500 oC for 3000 hours or longer, is generally hard to perform in the lab. Thus, the authors apply Van’t Hoff’s rule to evaluate the long-time, relatively low-temperature oxidation using rapid high-temperature oxidation. Based on Van’t Hoff’s rule, the oxidation at 650℃ for 5 minutes will show similar or more severe oxidation effects on the microstructure of Fe_73.5Si_13.5B₉Cu₁Nb₃ alloy after oxidation at 500 oC for 2730 hours. The microstructure evolution reveals that silicon and niobium in this alloy will quickly diffuse toward the samples’ surface during oxidation at 650 oC, and these two elements will form a dense layer to impede oxygen diffusion. Meanwhile, an α-Fe(Si) phase mainly composed of iron will be generated in the alloy, with its grain size slowly increasing during the oxidation process. Thermodynamic analysis indicates that the segregation of silicon and niobium can preserve the thermodynamic stability of the alloy system during oxidation and suppress the formation of intermetallic compounds during crystallization. The magnetic hysteresis loop results show that the coercivity of Fe_73.5Si_13.5B₉Cu₁Nb₃ alloy after oxidation at 650℃ for 5 minutes would stay at approximately 0.3 Oe, suggesting the Fe_73.5Si_13.5B₉Cu₁Nb₃ alloy might be a candidate for operating at 500 oC for more than 2700 hours. After that, its coercivity gradually increases to 61 Oe as the oxidation time rises to 0.5 hours, while its saturation magnetic induction intensity remains unchanged (~140 emu/g).