As a crucial component of data terminal acquisition devices, flexible strain sensor has shown promising applications in numerous fields, such as healthcare, bodynet, the intelligent traffic system, and the robotic system. For stretchable strain sensor, it remains a huge challenge to realize a fine balance of wide detection range and high sensitivity. Here, an electrically conductive carbon nanotube/thermoplastic polyurethane fiber with a multilayered, hollow, and monolith structure, accompanying high stretchability (up to 476% strain) and low density (about 0.46 g/cm3) is fabricated through a facile coaxial wet-spun assembly strategy. The as-prepared fibers with a designed independent sensitive zone and flexible supporting zone possess an ultralow percolation threshold (0.17 wt %) and a tunable size and structure. This structure endows the fiber with a good integration of adequate flexibility, suitable strength, and high elongation at break for wearable electronics. The fiber, which is then assembled as a strain sensor, realizes the perfect combination of the wide sensing range (>350% strain), high sensitivity (gauge factor (GF) = 166.7 at 350% strain), and excellent working durability (>10 000 cycles). Our sensor could also detect small compressing deformations (0.35% N-1 at 0.025-50 N) by capturing the resistance change of the fiber with superior stability. The highly stretchable, light weight, and multilayered fiber with the designed hollow-monolith structure provides a new route for the preparation of high-performance wearable electronics.