Constructing three-dimensional segregated graphene (3DG) network in polymer matrix is very effective in fabricating highly conductive polymer composites. However, it remains challenging to control the distribution of graphene with a concise and efficient method though the past efforts have been made. In this contribution, we propose a novel hydrothermal self-assembled strategy in combination of the post-impregnation process to prepare highly conductive polymer/graphene composites. Specifically, styrene-butadiene rubber/graphene nanocomposites with a three-dimensional graphene network (3DG-SBR) were prepared by employing the hydrothermal self-assembling, followed by impregnating by silver nitrate. The resultant graphene/styrene-butadiene rubber/silver particle (3DG-SBR-Ag) nanocomposite with 1.66 vol% of graphene exhibits the conductivity of 512 S m(-1), which is competitive to those prepared by template-guided assembly strategy. Impressively, the polymer composite possesses mechanical robustness due to the strong interfacial reaction between graphene and polymer matrix. This synthetic approach for graphene-based nanocomposites is generic, scalable, flexibility in complex geometry and cost-effective. The as-prepared products could potentially be applied as competitive candidates for practical application in electromagnetic wave absorption and electromagnetic interference shielding.