In order to research the mechanism of nano TiO₂ doping effect on micro-arc oxidation coating formed on aluminum alloy, it was treated by using micro-arc oxidation in different concentration of nano TiO₂ electrolyte. Surface morphologies,Ti, Al,O and elements content, phase composition of micro-arc oxidation coating were analyzed by SEM, EDS and XRD. Thickness and hardness of micro-arc oxidation coating and surface charge of nano TiO₂ in electrolyte were measured. The doping modification model was established in this paper. The results show that the oxidation voltage were raised at initial stage, and then declined after 5minutes.The numbers and sizes of hole on oxidation coating surface decreased, formation efficiency and compactness of coating, and the hardness of surface porosity coating increased. The nano TiO₂ was distributed homogeneous on oxidation coating surface, and inhomogeneous on cross-section. Oxidation coating was mainly composed by γ-Al₂O₃, mullite and little Si phase.