Abstract: With increasing of deposition machines dimension, the growth process of thin film material will show new phenomena. In order to get homogenous large caliber optical thin film, different single SiO2 layers were fabricated using ion-assisted electric gun evaporation under different ion source energy, deposition pressure, substrate temperature and different rotation speed. Spectroscopic ellipsometry and spectrometer were used to measure SiO2 samples ellipsometric parameters and transmission spectra, respectively. Refractive index and its distribution were got by fitting the corresponding measurement results. It is shown that rotation speed is the main cause for the inhomogeneity of large caliber SiO2 thin film. Ion source energy, deposition pressure and substrate temperature can modulate the inhomogeneity by affecting SiO2 thin films growing process. When the rotation speed is limited for large dimension optical elements, homogenous optical thin film can also be got by optimizing other deposition parameters.