Abstract: The influence of nitric acid treatment on the composition and structure of clinoptilolite (HEU) was analyzed by various characterization techniques, such as element analysis (ICP-AES), N₂ physical adsorption, X-ray powder diffraction (XRD), ²⁷Al/²⁹Si MAS NMR and DFT calculation.The Si/Al ratio and BET surface area of Na-K-HEU and NH₄-HEU were significantly increased after nitric acid treatment, and meanwhile, their relative crystallinity decreased to below 50% when the concentration of nitric acid reached 4 mol/L, illustrating the severe distortion of zeolite framework.However, the structure of H-HEU was more stable and it could maintain a high relative crystallinity of 94.8% even the concentration of nitric acid reached 6 mol/L.The results of ²⁷Al/²⁹Si MAS NMR indicated that some extra-framework aluminum species appeared in H-HEU and they could protect the framework aluminum from further distortion, whereas this protective effect did not exist on Na-K-HEU and NH₄-HEU, as they have little extra-framework aluminum species.It can be further proved that through back-exchanging the H-HEU to Na-HEU and K-HEU, their resistance to nitric acid treatment could be markedly improved.