Abstract: A ring-unit FSS is designed and put into absorbing materials to form the composite FSS absorbing structure, then the structure is numerically simulated through the spectral domain approach. The reflection coefficient of the structure at a microwave band in 2-16 GHz is calculated and the influence of ring-unit's sizes and aligning cycles on absorbing characteristics is studied. The results show that when the ring-unit FSS's unit spacing is 10 mm and unit size is 3.3 mm, the resonance frequency reflectivity decreases from -8.15 dB to -14.5 dB, and the absorbing band with whose reflectivity is larger than -5 dB broadens from 1.2 GHz to 3.05 GHz. Meanwhile, the resonance frequency reflectivity increases with the increasing of ring-unit size, and the absorbing material's bandwidth expands with the expansion of unit aligning cycles. Experimental results indicate that the FSS can be used in absorbing materials to improve its absorbing properties. Moreover, the required composite FSS absorbing structure can be obtained by adjusting the corresponding parameters, which expands the application scope of FSS in absorbing materials.