This study aims to characterize the structure of Lilii Lancifolii Bulbus polysaccharide (LP) and investigate the protective effect of the Lilii Lancifolii Bulbus water extract (LW) and LP on acute lung injury induced by PM_2.5 exposure in mice.LP was obtained by water-extraction and alcohol-precipitation method.The LP was separated and purified by cellulose chromatography,and its structure was analyzed by gel permeation chromatography and 1-phenyl-3-methyl-5-pyrazolinone (PMP) pre-column derivatization.More to the point,the effects of LW and LP on acute lung injury induced by PM_2.5 exposure in mice were investigated.The results show that total sugar content of LP was (70.98±2.36)% by phenol-sulfuric acid method,and the relative molecular weight of LP-1 was 5 906 Da,consisting of mannose and glucose in a molar ratio of 1.00∶2.01.In the pharmacodynamic study,compared with the control group,the weight increase of the model group was slow (P < 0.01),the lung tissue damage was serious,the lung index and the level of inflammatory factors in the lung tissue were increased (P < 0.01).Compared with the model group,the body weight of the mice in the positive drug (dexamethasone) group and the LW and LP groups increased significantly (P < 0.01),the lung tissue structure was relatively intact,and the lung index,lung tissue pathological score and inflammatory factor levels were significantly reduced (P < 0.05).In addition,at the same dose,the efficacy of LW and LP group were basically the same,indicating that the polysaccharide may be an important medicinal substances basis for Lilii Lancifolii Bulbus to effectively improve the lung tissue morphology of mice with lung injury,reduce the expression of inflammatory factors in lung,and play an important role in the therapeutic effect of acute lung injury induced byPM_2.5 exposure.