To explore the therapeutic effect of low molecular weight heparin calcium combined with letrozole for treating patients with infertility because of polycystic ovary syndrome (PCOS), and to study its impacts on the levels of plasma D-dimer (D-D) and fibrinopeptide A (FPA) of the patients. Methods: 90 patients with infertility because of PCOS were randomly divided into study group and control group from October 2020 to October 2022. The patients in the two groups received treatment with letrozole, while the patients in the study group received treatment of low molecular weight heparin calcium additionally. The therapeutic effect and the changes of the plasma D-D and FPA levels of the patients were compared between the two groups. Results: The total effective rate (93.3%) of the patients in the study group was significantly higher than that (77.8%) of the patients in the control group. The ovulation rate and the pregnancy rate of the patients in the study group after 3 and 12 months of treatment were significantly higher than those of the patients in the control group. The uterine artery resistance index (0.62±0.07) and the pulsatility index (1.54±0.26) of the patients in the study group were significantly lower than those (0.75±0.08 and 1.91±0.32) of the patients in the control group. The endometrial thickness (9.92±1.58 mm) of the patients in the study group was significantly higher than that (8.76±1.44 mm) of the patients in the control group. The levels of plasma D-D (0.32±0.09 mg/L) and FPA (8.52±2.39 mg/L) of the patients in the study group were significantly lower than those (3.56±0.48 mg/L and 11.71±3.30 mg/L) of the patients in the control group (P<0.05). There was no significant difference in the adverse reactions rate (17.8% vs 11.1%) of the patients between the two groups (P>0.05). Conclusion: Low molecular weight heparin calcium combined with letrozole for treating the patients with infertility because of PCOS has higher therapeutic effect, which can promote the ovulation and pregnancy and improve the plasma D-D and FPA levels of the patients, and with good safety.