In this study, a novel graphene/g-C3N4 microwave absorber is developed to solve the electromagnetic wave interference problem. Graphene/g-C3N4 composite is synthesized by loading g-C3N4 nanosheets on graphene through a simple liquid-phase approach. High-performance electromagnetic absorption performance can be achieved. The optimal reflection loss value is up to -29.6 dB under a thin coating layer of 1.5 mm. At the same time, the corresponding absorption bandwidth of this composite can reach 5.2 GHz (12.8-18 GHz). Excellent electromagnetic absorption property may be attributed to the current attenuation theory which has been proven by replacing graphene with porous graphene or graphene oxide. The results reveal that free electron numbers and loading mass of g-C3N4 on graphene play the key roles in the intensity of current attenuation and resistance value.