The invention provides a preparation method of a polymer composite fiber containing ultrafine metallic particles and the polymer composite fiber prepared by the preparation method. The method comprises the following steps: fusing and blending metal powder with thermoplastic polymer resin, directly longitudinally drawing when blend melt is extruded through an extruder and passes through an extruder finish mold, so as to obtain the polymer composite fiber containing the ultrafine metallic particles, wherein the metal powder is continuously crushed to form ultrafine metallic particles of which the particle sizes are relatively small in an in-situ manner through the action of polymers and metal in the process of forming the polymer fiber through the extruder finish mold, and is distributed into the polymer fiber as a disperse phase, so as to improve the properties of the composite fiber. The preparation method has good applicability and relatively low cost; and the particle sizes of the ultrafine metallic particles contained in the prepared composite fiber can be adjusted through adjustment on the particles size of the metal powder, the polymer variety and the like, or even can reach nanoscale, so that the composite fiber has excellent properties.