thermochromic vanadium dioxide (vo2) film is a potential material for smart-window. but the high refractive index ( > 2) of vo2 coating in visible band resulted in strong reflection and low visible transmission. if another coating with a definite low refractive index is laid on vo2 film to construct a gradient index multilayer system, the total reflection could be effectively reduced. therefore in this paper we designed a mesoporous sio2/vo2 double-layer system in which the mesoporous sio2 layer possessed an index adjustable from 1.243 to 1.354 to reduce the surface reflection of vo2-based smart window. the mesoporous structure has been investigated through several techniques including 2d gisaxs, tem and n-2 ad/desorption. more important, a facile, safe and low-cost solution method was employed to prepare vo2 film with 60 nm thickness directly from ammonium citrato-oxovanadate (iv) compound that was reported by us previously. on this newly designed mesoporous sio2/vo2 double-layer coating, the integral visible transmittance at 25 degrees c (t-vis,l) increased to 80.0% from 69.8% of pure vo2 coating while the corresponsive integral visible transmittance at 90 degrees c (t-vis,t-h) increased to 78.9% from 67.6% of pure vo2 coating, if the index of mesoporous sio2 layer was optimized to 1.299. simultaneously, the near infrared switching ability at 2000 nm (delta t-2000) reached 29.0% and solar energy modulation (delta t-sol) was maintained at 10.2%. the phase transition temperatures (t-t) for vo2 coating and the optimized sio2/vo2 double-layer coating were 51.9 degrees c and 53.8 degrees c respectively, far below 68 degrees c of bulky vo2. this optical performance should be very attractive for application in the further smart window because of little increased cost and greatly enhanced property.