Abstract: Objective To investigate the effect of low molecular weight heparin (LMH) combined with pulmonary surfactant (PS) on acute pulmonary embolism in young rats. Methods Forty-five 4-week-old male SD rats were randomly divided into control group, pulmonary embolism group, LMH group, PS group, LMH-PS group, 9 rats each. Except in the control group, the gelatin sponge particles were injected through jugular vein to make acute pulmonary embolism model. After the injection, LMH group was injected subcutaneously with 0.01 ml LMH /kg twice a day; PS group was perfused intratracheally with 120 mg PS/kg , once a day; LMH-PS group was treated with both LMH and PS. The control group was administrated the same volume of normal saline through jugular vein. On day 1, 4, 7 after injection, the arterial blood gas was analyzed, the pulmonary artery pressure (mPAP) and right ventricular pressure (RVP) were measured, the protein and gene expression levels of ET-1, MMP-9 were detected, and hematoxylin and eosin stain of lung tissue was performed. Results There were statistical differences in PaO2 among the 5 groups on day 1, 4 and 7 (P all < 0.05). On day 1 and 4, the level of PaO2 in LMH-PS was 85.4±6.8 mmHg and 87.8±4.7 mmHg which were significantly higher than in pulmonary embolism group, LMH group and PS group, but lower than in control group (P all < 0.05). There was significant difference in the mPAP on day 1 after injection between LMH-PS group and control group (P < 0.05), and then there were no differences on day 4 and 7 between two groups (P all > 0.05). However, the mPAP was still significantly lower in LMH-PS group than in other three groups (P < 0.05). The expression levels of mRNA and protein of ET-1 and MMP-1 on day 4 and 7 were statistically different among the 5 groups (P all < 0.05). Moreover, the expressions of mRNA and protein of ET-1 were lower in LMH-PS group than in other three groups except the control group. Conclusion LMH combined with PS could effectively treat acute pulmonary embolism in young rats.