Based on the equivalent transmission line theory, a three-layer tunable composite absorber with active frequency selective surface (AFSS) is designed. The first layer is an AFSS substrate. The middle layer is an AFSS, which is composed of a frequency selective surface (FSS) and a PIN diode array. The third layer is an insulator. Reflectance measurement shows that the reflection characteristic of the absorber can be dynamically adjusted by adjusting the bias voltage of the PIN diode array. When the bias voltage is 5 V, the best absorption property is observed. Reflection coefficient is below -8 dB and -10 dB at the frequency bands of 5—15 GHz and 5.3—13 GHz, respectively. Replacing PIN diodes with surface mount device (SMD) resistors, the reliability of the above results is further verified.