The core-shell composite nanomaterials composed of magnetic particles and dielectric materials have excellent microwave absorption properties and have become a research hotspot in the field of microwave absorption. The absorbing mechanism of different types of absorbers in composite materials is different, so their proportion has an important influence on the comprehensive absorbing performance. In this paper, a facile method is proposed to prepare core-shell FeBP@SiO₂ nanoparticles. This method combines chemical reduction and sol-gel to realize the controllable shell-core structure of composite particles. By changing the thickness of SiO₂ shell, the effect of shell thickness on microwave absorption performance was studied, and the microwave absorption mechanism was analyzed and explained. With the increase of SiO₂ shell thickness, the microwave absorption capacity of the particles increases first and then decreases. When the thickness of the SiO₂ shell is 38 nm, the FeBP@SiO₂ sample has a nice microwave absorption performance, and the reflection loss at a thickness of 2.19 mm obtains better absorption performance ( -52.66 dB ). This enhanced microwave absorption performance mainly comes from the new magnetic-dielectric interface, thereby improving the impedance matching and dielectric loss of the material. By designing the shell-core structure of the composite particles, the performance regulation of the composite absorber can be achieved. Therefore, this work might provide an important reference for the design of the next generation of new composite microwave absorbing materials.