Both morphological structure and chemical composition are indispensable factors for the design and fabrication of high-performance electromagnetic (EM) wave absorbers. Herein, we prepare a porous carbon composite material with high surface area by using recycled fluid coke as starting material via an alkaline activation process. The minimum reflection loss of the activated fluid coke (AFC-3.5) is -20 dB, while the effective absorption frequency band is 4.8-18 GHz. Ni nanocrystals subsequently grow in-situ and embed uniformly within porous framework of activated fluid coke to form porous carbon/Ni (C/Ni) composites. As a result, the lowest reflection loss value is further reduced to -47 dB, while effective EM wave absorption band (R-L < -10 dB) covers wider frequency ranges from 3.5 to 18 GHz with a low filler loading of 20 wt%. We also present the absorption mechanism of the porous structure and chemical composition. This work demonstrates the significance in exploring porous structures for the application of EM wave absorption, and for further research and development of novel EM wave absorption materials.