Niobium is an excellent material used for production of electrolytic capacitor, but when amorphous Nb2O5 dielectric film grows during electrolysis reaction meanwhile two kinds of low valence niobium oxide (NbO2 and NbO) forms, the presence of NbO2 and NbO has bad influence on the performances of niobium capacitor. In this work, through comparative experiments of amorphous film growth condition and analysis on the growth mechanism, the relative contents of three kinds of Nb oxides are well controlled, and performance of Nb capacitor samples has been improved greatly.