The polymer derived SiBCN has excellent electromagnetic wave absorbing performance, but the impedance matching characteristics of single SiBCN component are poor. The composite powder system of Mullite matrix and SiBCN absorbing agent was designed. The powders was uniformly dispersed in the water-soluble resin glue by ball milling method, the Mullite-SiBCN absorbing coating with different mullite content was prepared on the surface of epoxy resin plate by slurry brushing method. The reflectivity of the coating was tested by the bow method, and the effect of mullite content on the absorbing performance of the coatings was studied. The results showed that the absorption performance of the coatings first increased and then decreased with the mullite content increased. When the content is 70wt.%, the absorption performance of the coating is relatively the best, The frequency range of reflection loss less than-4dB can reach 3.5GHz. When the mullite content is lower than 70wt.%, the matching performance between the coating and the free space impedance was poor, most of the incident waves on the coating surface were reflected, thus when the mullite content exceeded 70wt.%, the SiBCN content of the coating was too low, resulting in insufficient microwave absorbing performance of the coating.