The last decade has witnessed a tremendous growth of research and development in flexible and wearable strain sensors. However, there are still some challenges to fabricate these strain sensors with high sensitivity, large workable range and low cost, etc. Here, we report a type of highly elastic strain sensor based on commercial Spandex fiber with conductive thermoplastic polyurethane (TPU)/multi-walled carbon nanotube (MWCNT) nanocomposite coating by layer-by-layer (LBL) method. The conductive nanocomposite coated fiber sensor exhibits excellent performance, including large workable strain (>200%), high sensitivity (gauge factor exceeds 150 in strain of 10~20%) and excellent repeatability. Besides, the presented sensor shows a regular frequency dependence within a measuring frequency range of 0.01~1 Hz, indicating which can be used as a probe for frequency detection. In addition, the effect of ultraviolet irradiation on its sensing performance is also investigated. It is observed that the sensitivity of sensor increases to some extent with increasing irradiation duration due to the presence of microcracks on the fiber surface under the stretching/releasing cycles, while a stable electromechanical response is still retained.