The thymus is located behind the sternum, close to the heart, grayish red, flat and oval, divided into two lobes, left and right, composed of lymphatic tissue. It is well developed before puberty and gradually degenerates after puberty and is replaced by adipose tissue. In the early embryonic stage, the thymus is formed by the epithelium of the ectoderm of the gill sulcus and the endoderm of the pharyngeal sac, so its early base is the epithelial tissue containing ectoderm and endoderm; after the migration of lymphatic stem cells, it gradually becomes a special lymphoid tissue, which has an important role in the cellular immunity of the body. The thymus is an important organ for the cultivation and selection of T cells. The lymphocytes in the blood, 70-80% of which are T-lymphocytes (T cells for short). They are originally tiny white cells growing in the bone marrow, which are sent to the thymus by the blood, nurtured by thymic hormones, and become mature, but not yet immune, T cells, which are then sent to the spleen, lymphatic system and other organs, where they are further grown under the influence of thymic hormones and ready to fight against various harmful enemies. Thymic hormones also increase the killing ability of lymphocytes and induce the maturation of B cells (also a type of lymphocyte). Thymosin and thymopoietin, which are secreted by thymic epithelial cells, both promote thymocyte differentiation. The thymus grows a variety of virgin T cells that are transported via the bloodstream to the surrounding lymphoid organs and lymphoid tissues. Thymopoietin produced by thymus is a protein and peptide hormone that stimulates the maturation of T lymphocytes and balances and regulates immune function, a hormone closely related to cellular immunity of the body. After puberty, the thymus gland gradually shrinks and its related functions are replaced by many secondary lymphatic tissue organs such as lymph nodes and spleen. The thymus is a central target organ and plays an important role in the pathogenesis of myasthenia gravis. Due to the occupation of the receptor by anti-acetylcholine receptor antibodies in the blood, it cannot bind to acetylcholine effectively; the complex of antibody and receptor with the participation of complement C3 can lyse the receptor; kill T cells and lymphokines and destroy the receptor; helper T cells can promote the synthesis of anti-acetylcholine receptor antibodies by B cells. Most of these antibodies are synthesized in the thymus, and T lymphocytes are also sensitized in the thymus, and the initiating antigens (i.e. myxoid cells) that can cause the autoimmune response in myasthenia gravis are also present in the thymus. This suggests that the thymus is the point of origin of myasthenia gravis. Today’s treatment targets are basically antibodies, lymphocytes and thymus tissue. The surgical resection of the thymus is one of the etiologic or primary treatments. In the last 20 years, thanks to the intensive research on the pathogenesis of myasthenia gravis and the development and application of standardized treatment, the mortality rate of the disease has been reduced from 35% 20 years ago to almost zero today. And 75%-85% of patients can achieve significant improvement or complete remission after surgical removal of the thymus gland plus comprehensive treatment [1].