Fiber-reinforced polymer composites (FRPCs) have gained wide applications in automobile, aircraft and electronic devices due to their superior strength to weight ratio, high fracture toughness, excellent anti-corrosion and ease of processing. With an annual production of millions of tons, the present state of the art in FRPCs critically limits the intrinsic versatility of fibers. For example, despite of owning high heat resistance of fibers, the heat deflection temperature of FRPCs is inevitably determined by the polymer matrix. Many filaments including carbon nanotubes, ceramic whiskers and metallic fibers behave themselves excellent thermal and electrical conductivity. However, due to the contact resistance between the filaments, present technologies may fabricate FRPCs with conductivities no more than 1% of the fiber itself. In this study, we show a low-cost and widely applicable strategy for “self-welding” the randomly dispersed short fibers to be a three-dimensionally continuous scaffold in the polymer matrix. This has been achieved by adding a small amount of “solder”, a third component capable of preferential segregation at the junction point of two fibers. To make the “solder” preferentially segregate on the correct position during compounding and forming process via traditional polymer processing machines, heterogeneity in chemistry or physics of the fiber surface and control of the interfacial affinity and rheological difference between the “solder” and the matrix polymer have been carefully investigated.