Abstract: 2.5D woven SiC_f/SiC composites were designed and prepared to meet the requirements of high temperature microwave absorbing structural composites, and the microwave absorbing properties were studied by combining experiment and simulation. The reflection loss of the material was measured by means of the bow method, and the geometrical parameters of the material were extracted by X-ray computed tomography (Micro-CT) to establish a full-thickness mesoscopic model. The reflection loss of the material was simulated and calculated on the CST electromagnetic simulation software, and compared with the experiment results. Based on the theory of equivalent electromagnetic parameters and field distribution map, the microwave-absorbing mechanism is analyzed, and the effects of geometric structure parameters, electromagnetic parameters, electromagnetic field polarization direction and incidence angle on the microwave-absorbing property of materials are studied. The experimental results show that the 2.5D woven SiC_f/SiC composites prepared in this paper have an effective absorption bandwidth of 3 GHz in the frequency range of 1-18 GHz, and the maximum reflection loss reaches −17 dB at the absorption peak of 9.3 GHz, which is basically consistent with the simulation results. The composite absorbs electromagnetic microwave mainly through the way of electrical loss, and its good microwave absorption performance is the result of the synergistic effect of structural design and material characteristics. The overall material thickness and fiber dielectric constant are the key factors affecting the microwave absorption performance of 2.5D woven SiC_f/SiC composites.