Neutralization process and mechanism of long highway tunnel shotcrete lining structure in general atmospheric environment which is contacted with nitrogen dioxide in automobile exhaust was analyzed. The neutralization experiment was carried out by nitric acid solution (pH=2) immersion method. Neutralization process of shotcrete was examined via physical and mechanical properties, damage depth and pH value and nitrate ion content of shotcrete. Afterwards, the mineral phases composition and microscopic morphology of neutralization zone were characterized using XRD, TG-DSC and SEM, which investigated the neutralization mechanism of shotcrete. The durability deterioration process of nitric acid exposed shotcrete included three steps of neutralization reaction, hydration products decomposition, and aggregate spalling. The pH value of shotcrete decreased while nitrate ion content increased. With increase of experiment aging, hydration products (calcium silicate hydrate and Ca(OH)₂) content decreased while corrosion products (Ca(NO₃)₂, calcium aluminate nitrogen hydrate and kaolinite) increased. Microstructure of neutralization zone was damaged, which results in a rapid decrase of the physical and mechanical properties.