High-entropy ceramics have exhibited promising application prospects in aerospace, electronic devices, and extreme environment protection. Current powder sintering route for preparing high-entropy ceramics is hindered by stringent powder requirements, reliance on long-term high-temperature and high-pressure synthesis, as well as compositional inhomogeneity and coarse grains. In this work, the low-temperature glass crystallization method was innovatively introduced into the preparation of high-entropy ceramics. Using garnet-structured rare-earth aluminates (RE3Al5O12, RE is rare-earth elements) as a model system, a series of single-phase RE3Al5O12 ceramics with entropy gradients were successfully synthesized through the glass crystallization method at a low temperature (1000 ℃). Notably, the as-prepared (Eu0.2Gd0.2Y0.2Yb0.2Lu0.2)3Al5O12 (HEC) samples exhibited a low thermal conductivity of 3.58 W m-1 K-1 (at 300 K) and a high thermal expansion coefficient (TEC) of 10.85×10-6 K-1, representing a 21% reduction in thermal conductivity and a 32% increase in TEC compared to reported Yb3Al5O12 ceramics. The HEC samples also exhibited superior mechanical properties compared to most existing high-entropy ceramics, with a hardness of 22.08 GPa and a Young’s modulus of 311.6 GPa. The exceptional comprehensive properties of the HEC samples make them a promising candidate material for thermal barrier coatings (TBCs) and high-temperature structural applications. This investigation confirms that high-entropy ceramics with outstanding properties can be successfully prepared using a glass crystallization method, providing a novel strategy for the low-temperature and pressureless controllable synthesis of single-phase high-entropy ceramics.