Based on the in-depth study of high-entropy alloys, a concept of configuration entropy stable single phase is introduced into inorganic non-metallic materials, thus developing high-entropy ceramics. The advantage of high-entropy ceramics is the diversity of composition and structure, which makes them have an application potential as functional materials. In this paper, high-entropy multiphase ceramics with a spinel structure and a perovskite structure were synthesized by a simple solid-state sintering method. The phase composition, microstructure, element content and valence state, and electromagnetic wave absorption properties were investigated. The microwave absorption properties of high-entropy multiphase ceramics sintered at different temperatures were analyzed. The results show that the high-entropy multiphase ceramics can be prepared, and two crystal structures (i.e., spinel structure and perovskite structure) can be formed due to the high-entropy effect. At 1300 ℃, there exists a maximum dielectric constant. The high-entropy multiphase ceramics prepared have the optimum electromagnetic wave absorption performance when the frequency range is 8.2–12.4 GHz.