Crack-free homogeneous nanoporous silica films on silicon wafers has been synthesized via a supercritical drying of wet gel films that were obtained by spinning coating the polymeric silica sol using acid/base two step sol-gel method with tetraethoxysilane(TEOS) as precursor. The nanoporous film is amorphous and three-dimensioned network, cross-linked by the primary particles which sizes distributed between 30～40nm showed by XRD and AFM. The thickness and refractive index of the silica films were measured by spectroscopic ellipsometry. The silica films' thickness is 300～1100nm, refractive index is 1.13～1.23, porosity is 50%～70% and dielectric constant is 1.9～2.4. The silica films' thickness increases as the solvent isopropanol (IPA) amount reduced, catalyst NH₄OH amount increased, SiO₂ sol viscosity increased and spin speed reduced. The films' dielectric constant loweres as NH₄OH amount and SiO₂ sol viscosity increases while IPA amount and spinning speed has little effect on the films' dielectric constant.