A 3D graphene network was constructed in polyamide 6 (PA6) monomers through the reduction and self-assembly of graphene oxide (GO), and then PA6 nanocomposites with low graphene content were fabricated through in situ polymerization. The effects of the 3D graphene network on the structure and properties of the PA6 were systematically investigated. Results show that the 3D graphene network can significantly improve the thermal conductivity of PA6. In the case of the PA6 with only 0.25 wt % graphene, its thermal conductivity is 0.69 W/(m K), about 2.88 times of that of the pure PA6. This improvement is attributed to the more-compact thermal conductive paths of the 3D graphene network, and its stronger interfacial interaction with PA6 in this work compares with those of pre-synthesized free-standing 3D graphene networks. Moreover, the mechanical properties and water resistance of PA6 also have significantly improved with the incorporation of the 3D graphene network.