Thermal effect in absorbing composite is concerned gradually due to electromagnetic environment tends to high power, high frequency and wide bandwidth. Microstructure characteristics and some basic material properties of carbonyl iron powder/silicone rubber (CIP/SR) are obtained by experiments method. Complex permittivity and permeability of CIP/SR composite as an absorbing material is measured by the vector network analyzer. CIP/SR composite with 78 wt% particles filling exhibits excellent absorption performance in C and X band. A numerical model coupled with electromagnetic wave, heat transfer and mechanics theory is presented to study the thermal effect and mechanical damage of absorbing composite under microwave radiation. Microwave heating and attenuation mechanism of CIP/SR composite are discussed. Electric field, temperature, stress and deformation in CIP/SR composite are quantitatively characterized. The results indicate temperature distribution relates to electric polarization and hysteresis loss. Temperature of CIP/SR composite increases by 36 degrees C as microwave radiation lasts 2 s at 9.37 GHz. Thermal expansion and stress concentration will be aggravated as frequency increases.