TiO2 is a promising high dielectric material which can be introduced into aluminum anode foils for realizing miniaturization, lightweight, and high quality of aluminum electrolytic capacitors because of its excellent permittivity. However, the complex porous framework of etched aluminum foil gives rise to the low step coverage and residual carbon impurities of TiO2 films, which can restrict the specific capacitance improvement. In this work, the TiO2 films were deposited by atomic layer deposition (ALD) using O3, O2 plasma (O2*), H2O as oxygen sources, respectively. The effects of oxygen sources on step coverage, carbon impurity content, crystalline structure, oxygen vacancies (OVs) concentration and band gap (Eg) of TiO2 films were studied. Besides, the capacitor performances of aluminum foils were investigated. The O3–TiO2 films showed the best conformability and the least carbon impurity content, while the O2*-TiO2 films exhibited the lowest step coverage due to the directionality of plasma. And the highest carbon impurity content in H2O–TiO2 films was caused by the poor reactivity of H2O. Moreover, the anatase TiO2 peak intensity of H2O–TiO2 films through sintering and anodization was the highest due to the highest deposited mass. The O3–TiO2 films were found to be optimally insulating by characterization of OVs concentration and Eg. Surprisingly, the composite dielectric films using O3 as oxygen source exhibited the excellent withstanding voltage of 21.9 V, while the specific capacitance of 159.9 μF cm-2 was maintained, and there were about 45.89% and 33.63% enhancements in the specific capacitance and CV compared with those without TiO2. These outstanding properties demonstrate that O3 is more favorable for depositing high-performance ALD TiO2 films on aluminum electrolytic capacitors.