The interfacial adhesion is a significant factor for improving mechanical properties of filler-reinforced polymer composites. In this work, the as-spun and hot-stretched poly(phenylene sulfide) (PPS) and multi-walled carbon nanotubes (MWCNTs) composite fibers were prepared. Tensile test showed that large enhancements in tensile strength and modulus were achieved in hot-stretched samples. The strong interfacial interaction leading to the effective load transferring was realized in hot-stretched composite fibers. Further analysis showed that the strong interfacial adhesion might be the result of the formation of pi-pi interaction between the benzene ring of PPS and the surface of MWCNTs with the help of hot-stretching, which could be the most important reason for the remarkable mechanical enhancement of the hot-stretched composite fibers.