The widespread application of 5 G wireless communication and flexible electronic devices has put forward new demands for electromagnetic (EM) absorbing materials. In this work, a flexible composite fabric was successfully prepared by using polyester (PET) knitted fabric as an elastic scaffold and subsequently anchoring reduced graphene oxide (rGO) through a simple impregnation-drying coating method. Regulating the concentration of graphene oxide (GO) impregnation solution can effectively control the rGO loading content for adjusting the EM parameters of PET@rGO composite fabric. When the concentration is 2.0 mg/mL, the as-prepared PET@rGO exhibits the optimum reflection loss (RL) value of −24.53 dB at 12.4 GHz and a corresponding effective absorption bandwidth (EAB, RL≤−10 dB) of 3.20 GHz (9.20–12.40 GHz). The impressive absorption property attributed to the synergistic effect of impedance matching and dielectric loss originated from the three-dimensional (3D) conductive network on fabric interlaced structures. More importantly, stretching PET@rGO composite fabrics to different deformations can dynamically adjust the EM absorption properties while maintaining the knitted structure. This work demonstrates an efficient avenue for the rational design of next-generation EM absorbing fabric, showing latent applications in flexible functional electronics.