Objective To investigate the degree of lung injury induced by different doses of particulate matter with diameters ≤2.5 microm (PM2.5) in asthmatic mice. Methods Male BALB/c mice were randomly divided into normal control group, ovalbumin (OVA) asthma group, 1, 5, 15 mg/mL PM2.5 treated OVA asthma group. The bronchoalveolar lavage fluid (BALF) was collected and the number of white blood cells was observed by Gimsa staining. ELISA was used to determine the concentrations of serum cytokines interferon γ (IFN-γ), interleukin 17 (IL-17) and IL-10 in the peripheral blood of mice. Real-time quantitative PCR was performed to detect the mRNA expression levels of Toll-like receptor 4 (TLR4), nuclear factor-κB (NF-κB) in peripheral blood mononuclear cells (PBMCs). The levels of T-bet, RORγt and FOXP3 were tested by Western blotting. The lung tissues were collected and the pathological changes were observed by HE staining. Results Compared with the normal control group, the OVA asthma group showed thickened alveolar septum, enlarged alveolar cavity and more obvious inflammatory cell infiltration, as well as significantly increased white blood cells associated with inflammatory response in the BALF. Whereas, compared with the OVA asthma group, the above mentioned changes in 15 mg/mL PM2.5 treated OVA asthma group were extremely obvious. ELISA showed that the levels of IFN-γ and IL-10 in the serum of the OVA asthma group were significantly lower than those in the control group, while IL-17 significantly increased. Compared with the OVA asthma group, the levels of IFN-γ and IL-10 in 15 mg/mL PM2-treated OVA asthma group significantly decreased, while the content of IL-17 significantly increased. Real-time quantitative PCR showed that the expressions of TLR4 and NF-κB in PBMCs of the OVA asthma group were significantly higher than those in the control group. In 15 mg/mL PM2.5-treated OVA asthma group, the levels of TLR4 and NF-κB increased significantly as compared with the OVA asthma group. Compared with the control group, the expressions of T-bet and FOXP3 proteins in the OVA asthma group were significantly lower than those in the control group, and the expression of RORγt protein was significantly increased. Compared with the OVA asthma group, the levels of T-bet and FOXP3 proteins were reduced a lot in 15 mg/mL PM2.5-treated OVA asthma group, while RORγt protein level was remarkably elevated. Conclusion The 15 mg/mL PM2.5 can obviously promote OVA-induced asthma and even lung injury by activating the TLR4/NF-κB signaling pathway.