Abstract: This article takes the CMSX-4 alloy, which is used for turbine blades, as the research object. The microstructure evolution and hardness changes of CMSX-4 alloy under overtemperature conditions were studied using the stereomicroscope, scanning electron microscopy, Vickers hardness tester, etc. The results show that as the temperature increased, the γ′ phase grew and accumulated, and resolve at the same time, while the γ′ phase cubic degree decreased, and the γ channel widened. When the overtemperature treatment temperature reached 1250 ℃, γ/γ′ phase interface became serrated. When the temperature reached 1300 ℃, the γ 'phases were all redissolved. When the temperature reached 1350 ℃, the alloy began to melt, resulting in a large number of holes and γ+γ′ eutectics. When holding at a certain temperature below 1300 ℃, with the extension of holding time, the percentage of γ′ phase gradually decreased and tended to be stable. When the temperature reached the re-dissolution temperature of the γ′ phase, the hardness of the specimen increases significantly due to the re-precipitation of small secondary γ′ phase, and the highest hardness reached 458 HV, which is about 12% higher than that of the original state. Therefore, for the microstructure damage and degradation caused by abnormal service such as overheating, the damage degree of the material can be evaluated by combining the macro morphology of the blades, morphology and percentage content of the γ′ phase, and hardness degradation.