Objective To discuss the ffect of fixation stiffness on fracture healing in rabbit bone lengthening,provide technical criterion and guidance for selecting the best fixation stiffness during the clinical bone lengthening. Methods During healing of bone lengthening,the model animals were fixed with external fixators. The different fixation stiffness were achieved by varying the diameters of the Kirschner pins (1.0mm and 1.5mm). The HE staining of the callus,X-ray images,and the load ratios of the treated leg to the control foot were obtained after healing. Ratios of the minimum to the maximum of equivalent bending moments of inertia on the lateral X-ray images were calculated with Self-developed software,and the total areas of the callus of the anteroposterior X-ray images were calculated. Finally,the relationship between these parameters and the fixation stiffness was analysis， and the effect of callus formation and fracture healing caused by different fixation stiffness was described. Results After the osteotomy,with the duration of treatment,the ratios of the moment of inertia of the equivalent weakest interface to the equivalent strongest interface on the lateral X-ray of the treated leg was reduced to 0.15±0.10,then gradually increased to 0.44±0.12. The load ratio of the treated leg to the control leg was increased from 0.49±0.04 to 0.81±0.06. The total area of the callus of the anteroposterior X-ray was gradually increased from 80.03mm2±50.04mm2 to 134.81mm2±39.73mm2. There were no statistically significant differences between the 1.0mm and 1.5mm diameter Kirschner fixation groups on the above three indicators.The total area of callus of the two groups showed callus area of 1.0mm Kirschner wire fixation group was slightly larger than 1.5mm Kirschner wire fixation group. The HE staining results of bone specimens of two groups showed that the activity of newborn bone tissue of 1.0mm Kirschner wire fixed group was slightly stronger than that in the 1.5mm Kirschner fixation group. Conclusions Both 1.0mm and 1.5mm Kirschner wires can heal the osteotomy end. The ratios of the moment of inertia of the equivalent weakest interface to the equivalent strongest interface on the lateral X-ray of the treated leg,and the load ratio of the treated leg to the control leg are no statistically significant differences between the two fixation groups. Using 1.0mm Kirschner wire fixation can make the osteotomy end produce more callus,and enhance the activity of new bone tissue.