Lightweight, stretchable, and flexible fiber-based triboelectric nanogenerators (TENG) offer great potential for self-powered sensing and energy harvesting in wearable devices and electronic textiles. However, the stability and durability of fiber electronics confront critical challenges of limited conductivity and stretchability. Herein, a fiber TENG device (FTENG) featuring superior conductivity and elasticity was developed by the rational design of a sheath-core structure on wet-spun elastomeric fiber substrates. Via evenly depositing silver nanoparticles onto styrene-ethylene-butylene-styrene/carbon nanotubes (SEBS/CNTs) fibers to form a deformable conducting inter-layer, electronegative polydimethylsiloxane (PDMS) is then encapsulated as both the protective sheath to enhance triboelectric charge generation and prevent silver oxidation and detachment. The developed sheath-core FTENG achieves ultra-high stretchability (>1260 %), high output voltage (19.4 V), and excellent cycling stability. Electronic textiles woven by the FTENG can efficiently harvest and store mechanical energy from human motion and the surrounding environments, which even demonstrates high sensitivity, excellent signal stability, and strong durability when applied as wearable triboelectric sensors, ensuring reliable self-powered human motion monitoring. This work provides an innovative strategy for advancing flexible electronic textiles in energy harvesting and self-powered sensing applications.