Abstract: AC impedance spectrum was used to study the electro-deoxidation reaction of silicon dioxide in the equalmolar molten salt CaCl2 and NaCl. The AC impedance spectra showed that the electro-deoxidation reaction of silicon dioxide is a two-step reaction and corresponding to the two reduction current peaks in the cyclic voltammetry curve. In the XRD pattern the intermediates SiO, CaSiO3 and Ca2SiO4 were found in the reaction system. By fitting the AC impedance spectra with equivalent circuits, the charge transfer resistance, double layer capacitance and activation energy during the reaction were obtained. In the low voltage range the charge transfer resistance and activation energy decrease with increasing cathodic voltage which implies that charge transfer is the rate control step of the electro-deoxidation of silicon dioxide. The results of the study are important to improve the current efficiency in the electro-deoxidation process of silicon dioxide.