Polyamide 6 (PA6)/graphene oxide (GO) nanocomposites were prepared through the in situ polymerization method. GO layers were partially reduced, exfoliated by intercalation of PA6 molecules with a high intercalation ratio, and dispersed uniformly in the matrix without obvious aggregation. The crystalline form of PA6 transformed from the gamma-form to the alpha-form by addition of GO, and the network structure centering on GO formed through the intermolecular interaction between the two phases. During the thermoaging process, compared with pure PA6, the reduced viscosity and tensile strength of the composite remained at a high level, and the carbonyl index increased much more slowly. In addition, the degradation temperature increased, the degradation rate decreased, and the activation energy changed slightly, indicating the enhanced thermal oxidative stability of PA6. With increasing GO content, the oxygen permeability coefficient decreased significantly, and the radical scavenging ratio increased, which was favorable for inhibiting the oxidative degradation of PA6 molecules.