The investigation of novel materials exhibiting exceptional resistance to calcium-magnesium-aluminum- silicate (CMAS) corrosion at temperatures of 1300 ℃ and above has emerged as a pivotal objective in the advancement of environmental barrier coatings for aircraft engines in recent years. In this study, atmospheric plasma spraying (APS) technology was employed to fabricate YAG(Y₃Al₅O₁₂)/Al₂O₃ coatings with eutectic composition, which was acknowledged as a promising material possessing outstanding CMAS corrosion resistance, thereby rendering it suitable for application in environmental barrier coatings. The as-deposited coatings were annealed at 1100, 1300, and 1500 ℃ to obtain different microstructures, and the corrosion resistance as well as mechanism of YAG/Al₂O₃ coatings against CMAS were investigated by comparing the corrosion results after exposure to CMAS at 1300 ℃. The reaction products between YAG/Al₂O₃ coatings and CMAS were found to be garnet-structure solid solution, CaAl₂Si₂O₈, and Ca₂MgSi₂O₇. The nearly continuous distribution of the garnet-structure solid solution layer at the reaction interface between YAG/Al₂O₃ coating annealed at 1100 ℃ and CMAS effectively impedes the diffusion of CMAS corrosion elements. For YAG/Al₂O₃ coating annealed at 1500 ℃, the increase in grain size and decrease in grain boundaries reduce the dissolution rate of the coating. Both of the above can affect the competitive precipitation of various products by influencing the ion transport rate in the corrosion process, and then improve the CMAS corrosion resistance of the coating. Moreover, heat-treatment temperature can tailor grain size, which influences both dissolution-precipitation rate and competitive precipitation of reaction products during CMAS corrosion. These findings provide guidance for selecting appropriate heat-treatment temperature and offer a novel approach to optimize CMAS corrosion resistance of YAG/Al₂O₃ coatings through microstructure optimization.