Based on in situ intercalation polymerization of aniline, a one-step synthesis of a graphene/Ag@PANI ternary composite is proposed. The results show that together with sunlight exposure, Ag+ induces the polymerization of aniline accompanied by self-reduction to form a Ag@PANI core-shell nanostructure, and consequently, exfoliates the graphite sheet into graphene. Through a PANI shell, Ag@PANI nanoparticles all anchor onto the surface of graphene, forming a stable ternary structure. The performance of graphene/Ag@PANI is closely related to its micro-morphology, which depends on the selected Ag+/aniline ratio during the synthesis. Double-layer absorbers with graphene/Ag@PANI as the absorbing layer present excellent absorption performance. The effective absorbing bandwidths of DB-10, DB-5, and DB-1 all exceed 3 GHz with a thickness of 1 mm and the reflection loss of 1.3 mm DB-10 reaches -44.5 dB at 10.5 GHz. The as-proposed facile and eco-friendly preparation of a graphene-based ternary composite is also of great significance for sensors, supercapacitor electronics, degradation of polymers, and other applications.