Abstract: High-performance graphene/epoxy nanocomposites are desired in many fields. However, the performance of graphene/epoxy nanocomposites is far from the actual demand, and many methods reported for the preparation of graphene/epoxy nanocomposites need the participation of environmentally harmful organic solvents. Herein, a novel, facile, and green cation–π interaction assisted phase transfer approach is developed for construction of graphene/epoxy nanocomposites. This strategy includes constructing potassium ion modified reduce graphene oxide (K-rGO) as nanofiller and choosing 1-(oxiran-2-ylmethyl)-1H-indole (IN) as phase transfer agent. The cation–π interaction is formed between K-rGO and indole group, which not only facilitates the transfer of K-rGO from water phase to organic phase, but also enhances the dispersion of K-rGO in epoxy. When the loading of K-rGO is 0.10 wt%, the tensile strength and elongation at break of epoxy nanocomposites increase by 116.6% and 126.5%, respectively, compared to neat epoxy. The simultaneous improvement in tensile strength and extensibility of graphene/epoxy nanocomposites is achieved. Furthermore, the prepared epoxy nanocomposites exhibit an excellent thermal conductivity owing to the good dispersion of K-rGO in epoxy matrix.