Al2O3-MoSi2 coating has excellent high-temperature stability. On this basis, Al2O3-MoSi2-Cu composite high-temperature absorbing coating was prepared by atmospheric plasma spraying method. The phase transition characteristics of Al2O3-MoSi2-Cu spraying feedstock under high temperatures were analyzed by thermogravimetric test, the phase analysis of coating was performed by an in situ XRD test at different temperatures, and the microstructure of the coating was characterized by SEM. The test results of high-temperature microwave absorption performance show that, in high-temperature air atmosphere, the Cu in the coating is gradually transformed into Cu2O by oxygen atom diffusion, and the microwave absorption performance of the coating gradually increases with the increase in temperature. The 1.7 mm-thick coating at 500 °C has the best absorbing performance with a reflection loss (RL) value of −17.96 dB and an effective absorbing bandwidth (RL < −10 dB) in X-band of 2.42 GHz. The prepared Al2O3-MoSi2-Cu composite high-temperature absorbing coating takes into account the dual advantages of high-temperature stability and high-temperature absorbing properties.

To obtain a high temperature-resistant microwave absorbing coating, an Al2O3–MoSi2/Cu composite coating was prepared by atmospheric plasma spraying. Compared with a normal temperature environment, there are a few reports on Cu as an absorbent for high temperature microwave absorbing coatings. Therefore, in this regard, wave absorbing property can be improved by using a Cu absorbent. The microstructure of a Al2O3–MoSi2/Cu coating was observed, and the dielectric properties of the composite coating in the high-temperature environment of the X-band were tested. The experimental results show that with the increase in temperature, Cu transforms Cu2O in the high-temperature environment and improves the coating’s wave absorption performance with MoSi2. In addition, a 1.4 mm-thick coating showed best microwave absorbing performance at 700 °C. The reflection loss was −19.09 dB, and the effective microwave absorbing bandwidth was 2.83 GHz (Reflection Loss < −10 dB). It was found that the Al2O3–MoSi2/Cu composite coating has good wave-absorbing performance in a 700 °C high-temperature environment.