Abstract: In pursuit of superior protection performance, the double-layer energy selective surface (ESS) has attracted significant research attention in recent years. However, most existing studies primarily focus on achieving lower insertion loss and higher shielding effectiveness, while neglecting critical investigations into the trigger thresholds of the double-layer structure and the shielding effectiveness during single-layer conduction states. These limitations hinder the design and practical implementation of double-layer ESS. This paper designs a double-layer ESS based on a single-layer "square-loop" structure and investigates its strong-field response characteristics under strong electromagnetic fields. Through analysis of insertion loss and shielding effectiveness across varying inter-layer spacings, we establish quantitative relationships between the trigger thresholds of each layer, the overall shielding effectiveness, and their single-layer ESS counterparts, while revealing variation patterns in the shielding effectiveness of double-layer ESS under different incident power levels. These findings provide theoretical support for designing and applying double-layer and multi-layer ESS configurations.