Abstract: Underground structures are widely applied as they are difficult to be located, relatively resistant to airbursts and ground explosions. However, the development of penetrating weapons gives rise to increasing threat to underground structures. Different from conventional rigid protection with structural strengthening, the effectiveness of flexibly protected underground structures with foam concrete as protection layers subjected to ground shocks were experimentally investigated in the present study. It is found that subjected to such loading, there are different situations. If the sum of energy input reduction due to dynamic soil-structure interaction and energy absorption via compression, both by applying foam concrete layer, is greater than that induced by the shortened standoff distance between the structure and explosion, the protection is effective. Otherwise, the protection is ineffective or even negative. For effective flexible protection of underground structures with foam concrete, major aspects such as the characteristics of ground shock, the compression strength and thickness of porous solid layers should be taken into account in design to ensure the sum of reduced energy input with dynamic soil-structure interaction and energy absorption by foam concrete compression being greater than the excessive energy input due to shorter standoff distance.