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Differential scanning calorimetry (DSC) and thermal simulation continuous cooling experiments were used to systematically study the behavior and microstructure of γ′ multi generations precipitation during the continuous cooling process in the third generation high performance PM superalloy. The results show that the alloy can obtain a multimodal size distribution of γ′ precipitates at cooling rates less than 1.4 ℃/s-1. The multi generations precipitation is closely related to the cooling rates. The slow cooling is the dominant factor causing the multimodal size distribution of γ′ precipitates. The linear regression method is used to get the quantitative relationship between the average sizes of secondary γ′ and cooling rates, and the formation kinetics and mechanism of multimodal size distribution of γ′ precipitates on the γ matrix are analyzed and discussed. Results of long-term aging of alloy with γ′ multimodal size distribution show that reverse coarsening phenomenon occurs due to the unstable morphology of the large-size secondary γ′, i.e., the average size of γ′ precipitates decreases with the increase of aging time. This reverse coarsening effect increases the strength of the alloy, and increases the long-term stability of the microstructure of the alloy. The conditions and reasons for the reverse coarsening of γ′ are also discussed. This work will provide a theoretical basis for the further research and development of new high performance PM superalloys.