In this paper, in order to advance the quality to withstand the high voltage requirement on the polyimide (PI) that is coated on silver-metalised low-emission glass with a slot-die method, the electrical breakdown strength was evaluated experimentally and theoretically on different thicknesses of PI coatings. With a similar direct voltage (DC) applied on the different thicknesses of the PI coatings, the thicker sample has less breakdown occurrence with a lower electric field. The defect areas in all samples lead to electrical breakdown in the lower applied voltage; it is illustrated and explained that the higher electric field was formed near the contamination area by simulating the electric-field patterns with a new two-dimensional finite-element analysis method.