Based on the thermochromic properties of VO2, wide applications such as smart windows have been investigated in the world. Using the sol-gel method, series of thermochromic films, including the pure and W-doped VO2 films, were fabricated on quartz substrate. WCl6 was used as the source of tungsten. Measurements of temperature dependent transmitting spectra indicated that obvious enhancement of transmitting variance between 30 and 90 degrees C can be realized through the addition of tungsten. In addition, based on the comparison of thermal hysteresis loops of the W-doped and undoped thermochromic films, the sharpening of temperature width and steepening of temperature-transmittance curves can be clearly seen from the W-doped thermochromic films, which indicated the improved phase change quality compared with the undoped ones. Therefore, the improvement of phase change quality has been realized successfully through the addition of tungsten compound into the thermochromic VO2 films.