An anti-loose interdental brush includes a plurality of fixing portions formed on a bristle stem of an interdental brush. Each fixing portion includes a bottom wall, a side wall formed laterally from the bottom wall, and a capacity area surrounded by the bottom wall and the side wall. The adjacent bottom walls of the adjacent fixing portions are orientated in different directions. Thus, the adjacent capacity areas are set in different directions to contact the handle. An outer diameter of the bottom wall can be preferably larger than or equal to an outer diameter of the bristle stem. Accordingly, a dual engaging and fixing effect between the bristle stem and the handle is attained to prevent the bristle stem from loosening and rotating due to an external force and to enhance safety in using.
