Abstract: Depositing protective coating on the surface of existing pressurized water reactor (PWR) fuel cladding is one of the important ways to develop accident tolerant fuel (ATF) cladding. In case of the lossofcoolant accident (LOCA), the protective coating can effectively reduce the reaction rate between cladding material and water vapor, reducing the hydrogen generation rate and maintaining the dimensional stability of cladding at the hightemperature environment. The medium entropy alloy (MEA) coating, a new concept of coating design is based on multiprincipal elements in equal or nearequal atomic ratio with less than five principal elements. In this work, the amorphous AlNbTiZr MEA coating prepared by magnetron cosputtering technology was vacuum annealed at 500 ℃ for different time. The vacuum heat treatment temperature is lower than the recrystallization temperature of Zr alloy matrix, which is determined to be 500 ℃. The isothermal annealing was performed at 500 ℃ for 10, 30, 60, 90 and 150 h, respectively. The vacuum heat treatment experiment was carried out on a self-made equipment consisting of static quartz tube and high temperature muffle furnace. The coating sample was placed in the quartz tube and vacuumed to below 1×10-3 Pa for sealing. Then the quartz tube was placed vertically in a high temperature furnace with PID temperature controller. The effect of isothermal annealing on microstructure evolution and mechanical properties of the coating were studied by XRD, SEM, nanoindentation and nanoscratch analysis. The results show that the phase structure of the coating appears crystallization after annealing for 150 h, but there is no crystallization at other temperatures. With the increase of annealing time, the amorphous peak tends to shift to a high angle. After isothermal annealing for 10 h and 30 h, the hardness and elastic modulus of the coating increase gradually. After isothermal annealing for 60 h, the hardness and elastic modulus of the coating reach the maximum of 10.7 GPa and 152.7 GPa, while the values decrease after 90 h and 150 h heat treatment. With the extension of isothermal annealing time, the plastic deformation ability and fracture toughness of the alloy are improved. The scratch test result shows that longtime annealing can effectively enhance the bonding strength between the coating and the substrate because the coating releases a large amount of internal stress and residual stress after hightemperature annealing, which increases the atomic density and greatly improves the plastic deformation ability of the coating. At the same time, after longtime annealing, the mutual diffusion of Al, Nb and Ti elements on the interface is significantly enhanced, and the interface bonding strength is significantly improved. These are conducive to the coating to adapt to the harsh working conditions of PWR.