The protective effectiveness of environmental barrier coatings (EBCs) for SiC-based composites is limited by the thickening and phase transformation of the SiO₂ scale, known as thermally grown oxide (TGO). In this study, a tri-layered TGO scale, comprising cristobalite, Hf-doped SiO₂ glass, and particle-reinforced Hf–Si–O glass, was formed during the oxidation of MoSi₂/HfO₂ duplex EBCs. The incorporation of gradient Hf doping and HfO₂/HfSiO₄ particle reinforcement effectively suppressed the crystallization and phase transition of SiO₂ and mitigated the internal stress within the EBCs, generating a crack-blocking effect. This effect prevented the scale of the TGOs from further channel crack propagation, enabling the SiC substrate with no detectable corrosion after 200 h of exposure at 1500 °C in steam, even when the TGOs thickness reached 24.5 μm. This work presents a novel strategy to simultaneously extend the service lifetime and enhance the high-temperature capability of EBCs through the tailored design of TGO composition and structure.