Microwave absorbing materials (MAMs) is a key component for weapon systems such as aircrafts, warships, and missiles to achieve the stealth performance by absorbing electromagnetic waves incident on. In this work, activated carbon fibers (ACFs) with average length of 1-3mm were prepared, and ACFs dielectric properties were investigated. The results show that ACFs have the frequency response effect, which is helpful for broadening the absorbing bands. According to ACF permittivity, the optimal design is carried out for four-layer microwave absorbing composites with thickness of 4mm based on the impedance matching design method. Following the optimization results the four-layer absorbing material was prepared, and it obtains a reflection loss below -10dB over 8GHz and the minimum value reaches -39.3dB in 2-18GHz.