Abstract: Dielectric properties of activated carbon fibers（ACFs） with average length of 1~2mm were investigated.The results show that the real part of the microwave permittivity rises,but the imaginary part falls with an increase in frequency,which indicates ACFs have the frequency response effect.The real part and imaginary part increase with increasing the ACF mass percentage in the composite.According to the above measured data,the optimal design is carried out for double-layer and four-layer microwave absorbing materials based on the impedance matching design method.According to the optimization results the four-layer absorbing material was prepared,and the band width with reflection loss below-10dB is obtained over 8GHz and the minimum value of reflection loss reaches-39.3dB.