In order to explore the effects of a combined exposure of formaldehyde (FA) and PM_2.5 with low concentration on the asthmatic mice, 70male Balb/c mice were randomly divided into 5groups:control group, OVA group, FA + OVA group, PM_2.5 + OVA group, FA + PM_2.5 + OVA group. Amongst 14mice in each group, 6were tested for airway hyper reactivity (AHR), and the remaining 8were detected for T-IgE in serum, as well as IFN-γ, IL-4in BALF, and ROS, MDA in lung tissue. Besides, the inflammatory cells in BALF were counted. In addition, H&E staining and immunohistochemical analysis of p-p38MAPK and p-p65NF-κ B were conducted with mice lung. A significant increase in the level of MDA (P<0.001), as well as a trend of increase in inflammatory lung cells(P>0.05) were demonstrated in asthmatic mice group exposed to 0.5mg/m³ FA alone. While in the group exposed to combined 0.5mg/m³ FA and 0.5mg/kg PM_2.5 with low concentration, a significant aggravation in lung inflammation(P<0.05, P<0.01) and attenuation in lung function(P<0.01) were observed. Significant increase was also found in the level of both oxidative stress and the phosphorylation of p38MAPK and NF-κ B in lung(P<0.05, P<0.001). In addition, the release of Th2 type cytokine was also enhanced significantly(P<0.01). Hence exposure to low-concentration FA alone could lead to the aggravation instead of inhibition of lung injury in asthmatic mice, and might further promote the damage to lungs in asthmatic mice caused by PM_2.5. In conclusion, combined exposure to FA and PM_2.5 with low concentration could lead to severe lung damage in asthmatic mice, with a possible correlation to the imbalance of Th1/Th2 type responses intensified by the ROS-mediated p38MAPK pathway.