The AC breakdown channel of multi-layer insulation was investigated with samples laminated between spherical electrodes under high and low values of applied field stresses. Both samples with butt-gap and without butt-gap immersed in liquid nitrogen (LN2) were used. In addition, the breakdown strength and the voltage-time characteristics of both types of samples were also determined. It was found from the experimental data that the buttgaps play a decision role in the shape and position of breakdown path as well as the magnitude of the breakdown strength and the lifetime index. However, creepage discharges at the interfaces between layers become more pronounced at a lower value of applied electric field stress. The degradation of insulation layers was found to depend on the value of the applied field stress as well as the sequence of layer in the multi-layer samples. Finally, the breakdown mechanism for multi-layer insulation was suggested.