Objective: To explore the effect of selenium supplementation on thyroid function, immune response and oxidative stress in HT patients with normal thyroid function. Methods: A total of 120 Hashimoto's thyroiditis patients with normal thyroid function admitted to our hospital from June 2016 to March 2018 were randomly divided into treatment group and control group with 60 cases in each group. The control group was given oral placebo, while the treatment group was given selenium yeast. The expression levels of thyroid function [including thyroid stimulating hormone (TSH), free triiodothyronine (FT3) and free thyroxine (FT4)], immune response [including thyroid peroxidase antibody (TPOAb), thyroglobulin antibody (TGAb), interferon-γ (IFN-γ), interleukin-4 (IL-4)] and oxidative stress [including malondialdehyde (MDA), glutathione peroxidase (GPx) and superoxide dismutase (SOD)] were observed and compared between the two groups. Results: The two groups showed significant changes in auxiliary diagnosis, immune response and oxidative stress. Before treatment, there was no significant difference in the levels between the two groups. After treatment, there was no significant difference in the above indexes between the control group and that before treatment. There was no significant difference in thyroid function related indexes between the treatment group and the control group. The levels of TPOAb, TGAb, IFN-γ and MDA in the treatment group were significantly lower than those before treatment and in the control group, while the levels of IL-4, GPx and SOD were significantly higher than those before treatment and in the control group. Conclusions: Selenium supplementation can stabilize thyroid function, reduce the level of thyroid antibody production, and improve immune response and oxidative stress response in HT patients.