Abstract: The technology of high power microwave（HPM） weaponry has been developed dramatically in recent years， which poses a severe threat to the electronic equipment based on high power electromagnetic pulse. Thus， it is of crucial significance to study HPM protection. Particularly， both protection against HPM and signal receiving and transmitting have long been a serious problem of “front door” protection for electronic equipment， and such a issue is solved in this paper by presenting and analyzing the energy selective HPM protection mechanism. A typical energy selective surface（ESS） and a kind of double-layer unsymmetrical ESS are presented， with their working performance being simulated， where more research on parameters that may influence the insertion loss and shielding effectiveness of the ESS structure are conducted. The results show that the transition loss in the transmitting mode of ESS is below 0.5 dB， and the shielding effectiveness in the protecting mode is over 20 dB within L-band. Thus， the ESS structure is characterized as a low-energy-pass filter， which essentially satisfies the requirements of the “front door” protection of electronic equipment. Overall， the equipment's performance can be improved by the double-layer unsymmetrical structure design.