Atmospheric particulate matter have potential toxic effects on the lung immune system, which break the balance of the immune system. The most harmful component of atmospheric pollution is atmospheric fine particulate matter (PM2.5). To explore the imbalance of Th1/Th2 differentiation triggered by PM2.5, the expression of T lymphocytes in lung tissue of mice was detected by immunohistochemistry, then flow cytometry was used to detect the proportion of Th1/Th2 in lung of mice after an intratracheal instillation of PM2.5. The immune histochemistry of lung tissue showed that there are a large number of CD4+ T cells in the infiltrating cell area, and the Th1 shifting in lung of mice was observed. The exposure to PM2.5 affects the immune balance, driving a Th1-biased immune response.