OBJECTIVE To explore the effect of low molecular weight heparin sodium on regulatory T cell(Treg) differentiation in rats with unexplained recurrent pregnancy loss(URPL) by regulating the miR-33a-3p, sphingosine-1-phosphate receptor 1(S1PR1) expression. METHODS Sixty female rats were randomly divided into blank control group(Control), negative control group(NC), URPL group, low molecular weight heparin sodium group(LMWH), and fingolimod(FTY720) group, with 12 rats in each group. On the 8th and 12st day of pregnancy, subcutaneous injection of anticardiolipin antibodies(ACA)-IgG into multiple parts of the back was used to induce rat URPL model. LMWH group and FTY720 group were administered on the basis of URPL group. On the 0th and 15th day of pregnancy, LMWH group rats were subcutaneously injected with low molecular weight heparin sodium(420 IU·kg^-1), while FTY720 group rats were injected with FTY720(100 μg·kg^-1) via tail vein, control group, NC group and URPL group rats were injected with an equal amount of physiological saline via the tail vein. After treatment, weigh the embryo and calculate the embryo absorption rate. HE staining was used to observe the pathology of placental tissue. Enzyme linked immunosorbent assay(ELISA) was used to detect interleukin(IL)-10 and transforming growth factor-β1(TGF-β1) levels in serum. Real time quantitative polymerase chain reaction(RT-qPCR) and Western blot were used to detect miR-33a-3p, S1PR1, forkheadbox protein 3(FOXP3), cytotoxic T lymphocyte associated protein 4(CTLA-4), and glucocorticoid induced tumor necrosis factor receptor(GITR) mRNA and protein levels in placental tissue. Flow cytometry was used to detect the number of Treg cells in placental tissue. RESULTS The placental cells of rats in Control group and NC group were arranged neatly, and structure was clear. Compared with Control group, There was a large amount of inflammatory cell infiltration, cell proliferation, and edema in placental tissue in URPL group, embryo quality was reduced, embryo absorption rate was increased, IL-10 and TGF-β1 levels in serum were decreased, miR-33a-3p, FOXP3, CTLA-4, GITR mRNA and protein levels in placental tissue were decreased, S1PR1 mRNA and protein levels was increased, the number of Treg cells was decreased(P<0.05). Compared with URPL group, the pathological damage to the placental tissue in LMWH group and FTY720 group were significantly reduced, with a small amount of inflammatory cell infiltration and edema visible, embryo quality was increased, embryo absorption rate was decreased, IL-10 and TGF-β1 levels in serum were increased, miR-33a-3p, FOXP3, CTLA-4, GITR mRNA and protein levels in placental tissue were increased, S1PR1 mRNA and protein levels was decreased, the number of Treg cells was increased(P<0.05). There was no statistically significant difference in the above indicators between LMWH group and FTY720 group(P>0.05). CONCLUSION Low molecular weight heparin sodium maybe promote Treg cells differentiation in URPL rats by regulating the miR-33a-3p, S1PR1 expression.