Frequency selective surface (FSS) is a two-dimensional array structure, which can be used to improve the performance of radar absorbing materials. We design circular unit of FSS, and give a simulation research on the circular unit of FSS with the spectral domain approach. The result shows that the circular unit of FSS greatly widens the bandwidth of radar absorbing materials, and decreases the reflectivity of resonance frequency; the bandwidth of radar absorbing materials increases with the increasing radius of the unit, and the resonance frequency drifts to low frequency, which provides theoretical support to the application of FSS in wave absorbing materials.