For optical system working in the polar environment, such as Antarctic Dome A, ITO film defrosting has outstanding advantage. The principle of ITO film defrosting is introduced. Present heating design for DC load with spot electrodes has several limitations: large area of local insufficient heating, electrode area overheating and uneven distribution on the whole surface. These affect imaging quality and defrosting effect. The evaluation index for defrosting effect is given, and then the new design of multiple-phase AC load with linear electrodes is put forward. It is analyzed using main path method. Targeting Antarctic Survey Telescope (AST), a 500 mm aperture mirror is studied using finite element method: each heating design is evaluated under equal power condition. The actual mirror is measured by infrared thermal imager. Using the new design, mean square deviation of mirror temperature reduces by 81.65% and peak to valley by 76.31%. The experimental results validate that the design of multiple phase AC load with linear electrodes greatly improves defrosting homogeneity, effectiveness and seeing. It will be applied to Antarctic optical telescope engineering.