Low thermal conductivity, compatible thermal expansion coefficient, and good calcium- magnesium-aluminosilicate (CMAS) corrosion resistance are critical requirements of environmental barrier coatings for silicon-based ceramics. Rare earth silicates have been recognized as one of the most promising environmental barrier coating candidates for good water vapor corrosion resistance. However, the relatively high thermal conductivity and high thermal expansion coefficient limit the practical application. Inspired by the high entropy effect, a novel rare earth monosilicate solid solution (Ho_0.25Lu_0.25Yb_0.25Eu_0.25)₂SiO₅ was designed to improve the overall performance. The as-synthesized (Ho_0.25Lu_0.25Yb_0.25Eu_0.25)₂SiO₅ shows very low thermal conductivity (1.07 W·m^-1·K^-1 at 600 ℃). Point defects including mass mismatch and oxygen vacancies mainly contribute to the good thermal insulation properties. The thermal expansion coefficient of (Ho_0.25Lu_0.25Yb_0.25Eu_0.25)₂SiO₅ can be decreased to (4.0-5.9)×10^-6 K^-1 due to severe lattice distortion and chemical bonding variation, which matches well with that of SiC ((4.5-5.5)×10^-6 K^-1). In addition, (Ho_0.25Lu_0.25Yb_0.25Eu_0.25)₂SiO₅ presents good resistance to CMAS corrosion. The improved performance of (Ho_0.25Lu_0.25Yb_0.25Eu_0.25)₂SiO₅ highlights it as a promising environmental barrier coating candidate.