Mechanically robust polymers are broadly applied engineering materials, in which internal, hidden defects resulting from mechanical, electrical and thermal fatigues are one of the major factors threatening their long-term stability. Self-healing of these defects requires the ability of spontaneous crack closure without manually holding the broken parts together, which is not accessible in mechanically robust polymers with previous approaches. Here, we report a self-healing polymer that can be reversibly switched between the glassy state and the high-elastic state, as enabled by the coexistence of reversible and permanent crosslinking sites in the synthesized dynamic crosslinker. This design allows for complete repair of internal defects for multiple times while maintaining the dimensional stability of mechanically robust polymers during the healing process. In addition, this self-healing material is readily prepared by curing the commercially available epoxy monomer with the synthesized dynamic crosslinker, which endows this approach with great potential for industrial applications.