Background Rats are often used to prepare skin defect models. However, the skin defect sizes of the models prepared by researchers are different, and the lack of consensus on the critical-size defect makes it difficult to compare their research results.Methods The time for wound closure was evaluated and recorded through gross observation. The regression equation between the healing time and the diameter of skin defect was established, which can be used to predict the healing time for a certain skin defect size in rats. Histochemical and immunohistochemical staining was used to observe the regeneration and reconstruction of skin appendages, and the functional skin repair was quantitatively scored.Results The critical-size defect of rats was determined based on the maximum capacity of structural skin repair, and the functional skin repair was quantitatively scored based on the regeneration and reconstruction of skin appendages. The allowable range of critical-size skin defect of SD rats lies between 45 and 50 mm in diameter. The concept of structural repair and the category of functional repair of injured skin are put forward. The regression equation between the structural skin healing time and defect diameters is established.Conclusion The allowable range of skin critical-size defect of SD rats lies between 45 and 50 mm in diameter. The regression equation between the structural skin healing time and defect diameters can be used to predict the healing time for a certain skin defect size in rats.