Ultrafine WO3−x (0 < x < 1) nanowires with aspect ratio greater than 100 were successfully synthesized by two-phase strategy. The crystal structure, morphology evolution and thermal stability of the samples were characterized by X-ray diffractometer (XRD), X-ray photoelectron spectrum (XPS), high resolution transmission electron microscopy (HRTEM), and thermal gravimetric analysis (TGA). The morphology of WO3−x nanowires is closely related to the pH value and oleamine content of the reaction system. By controlling the reaction conditions, a linear product with a diameter of less than 2 nm can be obtained, and the exposed active lattice plane is 110 crystal face. On this basis, the prepared ultrafine WO3−x nanowire sensor was tested for a series of harmful gases. The sensitivity of WO3−x-based gas sensors can reach 57.9 for 10 ppm NO2 at 100 °C, which is much higher than most reported values. The sensor has the excellent sensing performance of ppm NO2 at low operating temperature, which is due to its nanowire structure and oxygen vacancies. These results demonstrate that this type of sensor is a competitive candidate for NO2-sensing applications.