The work aims to obtain uniform, continuous and print-free micro-arc oxidation coatings on the surface of large-size TC4 alloy flat workpieces in the oxidation bath system of phosphate. Fully-closed and semi-closed cathodes were designed to perform oxidation test, and effects of different sealing methods on micro-arc oxide coating on titanium alloy were studied to determine final cathode sealing form. Specimens were subject to microarc oxidation, and morphology and composition of the coatings were detected with SEM/EDS. The fully-closed cathode could effectively avoid effects of weak electric field on the coatings. MAO coatings were affected by opening width and sealing direction of the semi-closed cathode. In the direction of opening, radiation area of the electric field was about 150 mm, the coating thickness decreased with the increase of the distance from the cathode. Thickness of the coatings attenuated slightly at 1 time of the cathode width, and the influence area was about 6 times of the cathode width. The influence area of oxidization would be 12 times of the cathode width if the cathode was not sealed on both ends. The cathode structure featuring in moderately opened forepart and fully-closed rear can control film falling off and ablation defect caused by change of the electric field intensity during the oxidation process, and continuous and uniform micro-arc oxide film is obtained. Oxidation process parameters can be set as voltage of 450 V, pulse width 15, pulse number 500 and scanning speed 10 mm/min.