Carbonyl iron powder (CIP) coated with a layer of epoxy (EP) shell, denoted as core-shell CIP@EP composites, were designed and prepared via in situ polymerization. The CIP@EP composites containing 4.5-6.8wt% EP were systematically characterized by X-ray diffraction, Fourier transform infrared spectroscopy, vibrating sample magnetometer, scanning and transmission electron microscopy, and vector network analyzer. The results indicated that CIP@EP composites with 9-254nm EP shell thickness were successfully prepared through an efficient in situ polymerization method. The particle diameter of core CIP particles was 0.49-4.24 mu m. The microwave absorption properties of the microspheres were then experimentally measured, and the CIP@EP composites exhibited a maximum reflection loss value of -66.2dB at 7.1GHz at 2.0mm absorber thickness. The effective absorbing bandwidth below -10dB was 8.0GHz (from 10.0 to 18.0GHz). The presence of the EP shell not only enhanced the microwave absorption performance of CIP@EP composites but also improved the overall chemical stability of CIP particles. The as-prepared CIP@EP composites may be a promising candidate for electromagnetic wave absorption applications, and the core-shell structure design can be extended to other microwave absorption materials.