Porosity defects such as internal cracks occur during the production process of heavy forgings, and the existence of such defects has major safety hazards. Therefore, the internal cracks of low-carbon steel were repaired by high temperature thermal plastic deformation, the evolution condition of microstructure in the internal crack repair area was analyzed by SEM, metallographic analysis and other methods, and the impact performance recovery condition was studied by standard Charpy experiments. The results show that in the repair process, the internal cracks disappear completely by only one upsetting, but the impact performance is only partially recavered. However, through the two upsetting and one drawing process at a higher temperature, while the internal cracks disappear completely, the impact performance is also fully recovered, which is related to the grain distribution in the internal crack repair area. In addition, after different thermal plastic deformation processes, the segregation of Al and Si  elements is observed in the internal cracks repair area, and the results lay the foundation for the establishment of internal cracks repair process standard for heavy forgings.