Objective To explore the influence of thyroid dysfunction in the first half pregnancy (≤20 gestational weeks) on gestational diabetes. Methords By adopting the method of prospective intervention study, general data of pregnant-women in the care clinics of our hospital were collected. Thyroid function was detected and evaluated by ATA criteria. All cases were divided into two groups: normal thyroid function group and thyroid dysfunction group. The thyroid dysfunction group was intervened. 75 g OGTT were conducted in all cases in 24-28 gestational-weeks. GDM was diagnosed by IADPSG diagnostic criteria. GDM prevalence was compared between normal thyroid function group and abnormal thyroid function groups. Results 1 062 cases of pregnant women were collected. 857 cases of pregnant women were normal, while 14 cases were subclinical hyperthyroidism (1.32%), 164 cases were subclinical hypothyroidism (15.4%), 22 cases were hypothyroxinemia (2.07%). Compared with normal thyroid function, prevalence rate of GDM was the highest in hypothyroxinemia group (54.5% vs 27.8%). There was no significant difference between subclinical hypothyroidism group and normal thyroid function group. Conclusion Thyroid dysfunction in the first half pregnancy is associated with GDM , Screening and treatment of pregnant thyroid dysfunction should be attached.