Engineering stoichiometry of materials is an effective strategy to optimize the comprehensive properties. Here we prepare high-entropy Er2(Y0.2Yb0.2Nb0.2Ta0.2Cex)2Oδ with single-phase fluorite structure by a solid-state reaction. The effects of stoichiometry of cerium on thermophysical properties and corrosion resistance to calcium−magnesium−alumina−silicate (CMAS) of the high-entropy oxides have been investigated. By engineering the cerium stoichiometry, thermal conductivity increases slightly from 1.21 to 1.36 W·m−1·K−1 and thermal expansion coefficients also increases from 10.56 × 10−6 to 11.61 × 10−6 °C−1 at 1000 °C. Furthermore, corrosion resistance of the high-entropy Er2(Nb0.2Ta0.2Y0.2Yb0.2Cex)2Oδ improves with the increase of cerium content. The effects of cerium content on CMAS resistance have been discussed in detail. Our work reveals that engineering stoichiometry of high-entropy oxides is expected to be an effective strategy to broaden the compositional space and optimize the thermophysical properties and CMAS corrosion resistance.