WO3 with a mixed phase (m-WO3 and h-WO3) was synthesized by facile hydrothermal and annealing methods using biomass carbon (Hemp stems) as a sacrificial template. The biomorphic hierarchical structure effectively enhanced the dispersion of WO3 microspheres, which increased the contact area for the target gas molecule. The control of the mixed phase compositions was achieved by space confinement of biomass carbon and heat treatments. The synergistic effect of the mixed phase increased the mobility of carriers and promoted the separation and transport of electrons and holes. Thus, it improved the adsorption-diffusion capacity of NO2 molecules and enhanced the sensor performance at lower operating temperature effectively. The B-WO3-04 (450 °C, 4 h) exhibited an ultra-high response (Ra/Rg = 71.07) towards 100 ppm NO2 gas with excellent repeatability and appreciable long-term stability at room temperature. The significant improvement of B-WO3-04 gas sensing performance was mainly due to its unique hierarchical structure and the optimal proportion of the mixed phase composition.