Melt intercalation has typically been used to modify polymers with layered nanoparticles to obtain excellent performance. Polyphenylene sulfide (PPS)/graphene nanoplatelets (GNPs) composites were manufactured by one-step melt intercalation under shearing force. The morphology, thermal properties, and mechanical properties of the PPS/GNPs composites were then studied in detail. Graphene oxide showed poor thermal stability compared with GNPs. The GNPs were well dispersed in the PPS matrix, with a slight aggregation. Furthermore, the addition of GNPs accelerated the crystallization of PPS and reduced the incomplete crystallization of PPS due to the heterogeneous nucleation of GNPs. The thermal stability of the PPS/GNPs composite showed a significant improvement because of the good thermal conductivity and mass transport barrier effect of GNPs. The temperature corresponding to the heat-resisting index (THRI) of the PPS/GNPs composites was also significantly improved due to the addition of GNPs. When the content of GNPs was only 0.5 wt%, the tensile strength and tensile modulus of the PPS/GNPs composites significantly increased compared with pure PPS; this could be attributed to the heterogeneous nucleation of nanoplatelets and the interfacial interaction between PPS and GNPs. The dynamic mechanical analysis of the PPS/GNPs composites indicated that the storage modulus of the PPS polymer increased significantly due to the addition of GNPs.