The work aims to study the failure process of a composite coating system composed of zinc yellow epoxy primer and acrylic polyurethane topcoat used in 5A06 aluminum substrate. The laboratory cyclic accelerated test with a cycle of “UV/condensation for 3 days, neutral salt spray for 3 days and low temperature exposure for 1 day” was designed. The performance of the composite coating was studied by adopting AC impedance spectroscopy and measuring gloss, color difference and IR. At 16 cycles of the cyclic accelerated test, the gloss loss and color difference of the topcoat increased to the level of slight loss of gloss and slight discoloration. The surface morphology of the topcoat and the impedance module changed obviously. At 12 cycles, small blister was observed obviously in the optical microscope, but the impedance module at 0.01 Hz of the coating remained above 109 ??cm2. From 12th cycles, the number of blisters increased and some of broken blisters caused the out flowing of pigment. At 14 cycles, the impedance module at 0.01 Hz lowered to 108 ??cm2. The blisters continuously increased and part of them peeled off. At 16 cycles, the impedance at 0.01 Hz lowered to 107 ??cm2. The breakage of characteristic functional groups and polymer chains of the resin matrix of acrylic polyurethane topcoat, and incompleteness of the coating may be caused by UV irradiation and accelerate the permeation of the corrosive medium such as water, oxygen and corrosive chloride, thus resulting in coating failure.