Objective:To analyze the effect of different doses of low molecular weight heparin combined with nifedipine on se-vere preeclampsia and its effect on oxidative stress in patients.Methods:A total of 102 pregnant women with severe preeclampsia were included and divided into high-dose group(LD group)and low-dose group(SD group)according to different treatment methods,with 51 cases in each group.Both groups were treated with low molecular weight heparin combined with nifedipine on the basis of convention-al treatment,among which the SD group was treated with low molecular weight heparin(2,500 U/time).LD group was treated with high dose of low molecular weight heparin(5,000 U/time).After 1 week of treatment,the clinical efficacy,oxidative stress indicators[superoxide dismutase(SOD),malondialdehyde(MDA),glutathione peroxidase(GSH Px),and protein oxidation products(AOPP)],levels of placental hypoxia cytokines[placental growth factor(PLGF),pregnancy associated plasma protein A(PAPP-A),and soluble endothelial factor(sEng)],and pregnancy outcomes were compared between the two groups.Results:The total effective rate of LD group and SD group was 92.16％and 76.47％,respectively,and LD group was higher than SD group(P＜0.05).There were no significant differences in oxidative stress level and placental hypoxic cytokine expression between the two groups before treat-ment(P＞0.05).SOD,GSH-Px,PLGF and PAPP-A of the two groups after treatment were higher than those before treatment(P＜0.05),and LD group was higher than SD group(P＜0.05).After treatment,MDA,AOPP and sEng in two groups were lower than be-fore treatment(P＜0.05),and LD group was lower than SD group(P＜0.05).The incidence of adverse pregnancy outcomes in LD group and SD group was 11.76％and 29.41％,respectively,and LD group was lower than SD group(P＜0.05).Conclusion:Low molecular weight heparin combined with nifedipine can be effectively applied in the treatment of severe preeclampsia,but the efficacy of different doses of low molecular weight heparin is different.Large doses of low molecular weight heparin are more conducive to impro-ving clinical efficacy,improving maternal oxidative stress and placental hypoxia,and reducing the incidence of adverse pregnancy.