Polymer composite fibers with superior properties such as excellent combined strength and toughness and biocompatibility can be used in high-tech applications of braided protective devices and smart wearable, however the research of high-performance polymer composite fiber remains in the infant stage. Here we present a strategy to produce strong and tough anisotropic polymer nanocomposite fibers with orientedly aligned salt rods using mechanical stretching-assisted salting-out treatment. The prepared nanocomposite fibers have a tensile strength of up to 786 ± 2.7 MPa and an elongation at break of 81%, and the anisotropic fibers exhibit good transmission of mechanical vibration in the longitudinal direction with high resolution. During the fabrication process, the salt builds up into oriented rods during the directional salting process, and the polymer is confined to the 150 nm domain between the rods after the solvent is completely evaporated, giving the nanocomposite fibers superior mechanical properties. The presented strategy can be applied to the continuous mass production of nanocomposite fibers and is also generalizable to other polymer nanocomposites, which could extend the applicability of nanocomposite fibers to conditions involving more demanding mechanical loading and mechanical vibration transmission.