Thermal barrier coatings (TBCs) are critical in protecting hot-section components. A double-layered TBC system comprising a eutectic Zr–Ta–O (ZTO) core–shell structured top layer and a yttria-stabilized zirconia (YSZ) underlayer was fabricated via atmospheric plasma spraying (APS). This study systematically investigates its mechanical properties and corrosion resistance under calcium–magnesium–aluminosilicate (CMAS, CaO–MgO–Al₂O₃–SiO₂) attack. The results demonstrate that the eutectic microstructure exhibits exceptional plastic deformability, achieving a compressive strain of more than 30% and a yield strength of up to 4.5 GPa through in situ mechanical testing. The dense Zr–Ta–O layer effectively seals CMAS infiltration through eutectic solidification-induced densification. Simultaneously, it functions as a sacrificial layer where phase transformation and thermal expansion mismatch induce strain, triggering spallation of corrosion products to protect the underlying YSZ. Finite element simulations quantitatively reveal the distribution of interfacial stress fields governing CMAS-driven crack propagation at the top layer. This design paradigm provides new insights into CMAS-resistant eutectic TBC architectures.