The invention discloses an application of MBL in preparing a drug for preventing or treating disease taking Tregs as a target spot and belongs to the technical field of a novel application of mannan binding lectin. The key point of the technical scheme is as follows: the invention relates to the application of MBL in preparing a drug for preventing or treating infectious diseases, autoimmune diseases or/and allergic diseases induced by taking Tregs as the target spot, wherein MBL realizes functions of the drug by accelerating induced differentiation of a CD4<+>CD25-T cell toward a Tregs cell. Through exploration, a novel factor of adjusting induced differentiation of the Tregs cell is obtained, that is, MBL directly enhances induced differentiation of the CD4<+>CD25-T cell toward the Tregs cell. The application disclosed by the invention highlights a novel function of MBL, and MBL not only plays an important anti-infection role in inherent immunity, but also can be a pertinent target spot for treating various autoimmune diseases and inflammatory diseases in intervention of induced differentiation of the Tregs cell.