A kind of elastic, conductive polyaniline/polyurethane (PANI/PU) fiber was prepared by in-situ chemical oxidative polymerization of PANI on PU fiber surfaces. The conductivity for the resulting fibers exhibited a maximum value of 10 (Ω·cm) while the highly elastic nature of the PU matrix remained. Scanning electron microscope images indicated that PANI was located on the surface of the PU fibers with an interpenetration interface due to the penetrating of aniline monomers into the PU matrix. Importantly, the interaction between PANI layer and PU matrix greatly benefited their highly structural stability. The piezoresistivity of the resulting fibers was identified in a high strain deformation range up to 1500%. The average strain gauge factor between 0% and 400% of the fiber was about 3, which is 1.5 times higher than the normal polymer strain sensors. Furthermore, the resistance response was reversible for the fibers on cyclic force loading, which opens a high opportunity for their being used as a strain sensing element in smart clothing.