Severe atmospheric haze caused by industrial pollution has severely affected human health and led to the increasing incidence of cardiopulmonary diseases, including pneumonia. Conventional methods for diagnosis of pneumonia are complicated and tedious, and current clinical imaging techniques might cause organ injuries to some extent. Therefore, an accurate, fast, and intact imaging method must be developed to diagnose pneumonia in the early stages. In this study, we propose a new two-photon fluorescence probe, named as ASPC, for detection of the activity of the inflammatory biomarker LTA4H through specific recognition and cleavage of amides containing the unnatural amino acid l-AspBzl. The activity of LTA4H in the lung tissues of mice was rapidly and accurately monitored for the first time and could be an indicator for diagnosis of pneumonia. The severity of pneumonia in mice caused by haze particulate was determined through imaging the activity of LTA4H as biomarker and confirmed using a commercial ELISA kit of interleukin-1β. This work provides a promising method for clinical detection of pneumonia and for screening specific depressors of LTA4H as potential drug candidates.