To control structure of nano-porous silica films effectively, a base/acid two-step catalytic sol-gel process was investigated. The nano-porous structure and IR absorption of the thin films were characterized by means of FE-SEM. AFM, SEM, FTIR, and ellipsometry, respectively. The experimental results have shown that the base/acid two-step catalysis can adjust the refractive index of the films from 1.18 to 1.41 rapidly and continuously. With a change of the experimental conditions., the frequency shift of the omega (4) (TO3) peak of FTIR absorption to a lower wave number indicates a decrease in the average Si-O-Si bridging angle. A reduction of FWHM of the omega (4) peak reveals a narrow distribution of the Si-O-Si bridging angle. These results suggest that influence of the experimental conditions on the FTIR absorption of the nano-porous silica films is attributed to a change of silica particle shape and state of aggregation of the particles in the films.