Stretchable and sensitive fiber-shaped strain sensor with stable sensing performance is highly desirable for wearable electronics. However, it is still a challenge to simply and economically fabricate such strain sensors in large scale for practical applications. Herein, we report a strain sensor based on a thermoplastic polyurethane (TPU) layer encapsulated core-sheath single-walled carbon nanotube (SWCNT)-reduced graphene oxide (RGO)/PU composite fiber through a dip-coating process. The synergistic effect between SWCNT and RGO contributes to the formation of a highly sensitive and conductive layer on elastic PU core, while the outmost TPU layer protects the conductive layer against abrasion or delamination. The results demonstrate that the fiber sensor with 50 wt% RGO in the conductive layer simultaneously exhibits a wide sensing range of 465%, a high gauge factor (GF) up to 114.7, and good cyclic stability for more than 1000 stretching cycles. Importantly, the fiber sensor shows high stability with little change in conductivity even by harsh treatment in ultrasonic bath for 250 min. The applications of our fiber sensor in monitoring human motions like elbow bending, phonation, pulse, and underwater sensing are also demonstrated. The reported fiber strain sensor provides a good candidate for next-generation intelligent wearable devices.