In this paper, we propose a flexible energy selective surface (ESS) for adaptive high-power microwave (HPM) protection. The unit cell of ESS consists of a square ring, a metal disc, a T-shaped metal structure, and a cross-toe structure, where PIN diodes are loaded at the gaps of the T-shaped metal wires. The insertion loss (IL) of ESS is less than -0.5 dB and the shielding effectiveness (SE) is greater than -10 dB in the same band covering from 1.5-2.6GHz. Then, we combine the equivalent circuit model (ECM) to analyze the physical mechanism of ESS. Additionally, we also simulate the different curvature cases of ESS, where the simulated results show that the proposed flexible ESS maintains high performance when the curvature of ESS reaches 90°.