In this study, a flexible highly sensitive strain sensor was fabricated using thermoplastic polyurethane (TPU) and carbonized woven fabric based on polyacrylonitrile nanofiber yarn (PNY). The carbonized PNY fabric was prepared according to the following steps: filaments fabricated by electro spinning were twisted into a yarn; the yarn was treated by a sizing agent and weaved as a weft yarn; subsequently, the woven fabric was washed, pressed, and dried; finally, the carbonized PNY woven fabric was obtained through stabilization and carbonization. The effects of the thickness of the TPU film and the structure of the fabric on the sensing properties are discussed. The flexible strain sensor exhibits excellent sensitivity in the high-sensing strain range (average gauge factor = 77.3 within 12% strain) and high durability and stability (more than 1000 stretching cycles at 5% strain). Moreover, the strain sensor exhibits an excellent linear relationship between the strain and relative resistance change and accurately detects a full range of human activities (both vigorous and subtle). Such flexible highly sensitive strain sensors can be easily incorporated onto the surfaces of textiles and within electronics for use in various applications, toward applications such as smart textiles and health monitoring.