Objective: To explore the effect of hyperbaric oxygen preconditioning with different frequency on the survival rate of flap and ischemia-reperfusion injury in rats after transplantation, and to explore the best preconditioning conditions to improve the survival rate of rat flaps after transplantation. Methods: Thirty-six Sprague Dawley rats were randomly divided into four groups according to the random number table method, 9 groups in each group.Four groups of rats were pretreated with hyperbaric oxygen pretreatment for 0, 2, 4, and 6 days before the operation, control group, pretreatment 2 d group, pretreatment 4 d group, and pretreatment 6 d group. Taking the midline of the back of the rat as the axis, an ultra-long random flap with a pedicle at the tail end and about 1 cm from the superior iliac spine was designed and cut to a size of 10.0 cm×2.5 cm. The survival of the flaps in each group was observed and the final survival area and survival rate of the flaps were measured on the 7th day after surgery. On the 7th day after operation, the tissue was taken at a distance of 5 cm from the pedicle, and the histopathology was observed; The content of superoxide dismutase (SOD) and malondialdehyde (MDA) in flap tissue was detected by immunohistochemistry, and the expression rate of positive cells in each group was calculated. Immunofluorescence was used to detect the expression of interleukin-6 (IL-6) in the flap tissue. Results: On the 7th day after the operation, the survival area and survival rate of the transplanted flaps in the hyperbaric oxygen pretreatment group were significantly higher than those in the control group (P<0.05), and the pretreatment 4 d and 6 d groups were significantly higher than the pretreated 2 d group (P<0.05), but there was no significant difference between the pretreated 4 d group and the 6 d group (P=0.095). Pathological observation on the 7th day after operation showed that there was some necrosis in the control group, the vascular cells in the pretreated 2 d group had more vascular structures, and more neovascularization was observed in the pretreated 4 d group. The inflammatory cells were the least in the 6 d pretreatment group, and the neovascularization was the same as the pretreatment 4 d group. The absorbance A value of SOD in the control group was 0.009 7±0.000 3, and the positive expression rate was 20%, which was significantly lower than that in the hyperbaric oxygen pretreatment group. The difference was statistically significant (P<0.05). However, the absorbance A value of MDA in the control group was 0.055 1±0.003 0, and the positive expression rate was 55%, which was significantly higher than that in the hyperbaric oxygen pretreatment group. The difference was statistically significant (P<0.05). Among them, the SOD absorbance A value of the pretreated 2 d group was 0.023 8±0.003 0, and the positive expression rate was 30%, which was lower than the pretreatment 4 d group (absorbance A value 0.046 9±0.003 0, positive expression rate 35%) and 6 d group (absorbance A value 0.047 2±0.003 6, positive expression rate 40%), The MDA absorbance A value of the pretreated 2 d group was 0.037 2±0.003 2, and the positive expression rate was 30%, which was higher than the pretreatment 4 d group (absorbance A value 0.014 7±0.002 4, positive expression rate 5%) and 6 d group (absorbance A value 0.017 0±0.001 8, positive expression rate 10%), the difference was statistically significant (P<0.05). However, there was no significant difference in the expression of SOD and MDA between the pretreated 4 d group and the pretreated 6 d group (P>0.05). The expression of IL-6 in the hyperbaric oxygen pretreatment group was significantly lower than that in the control group (absorbance A value 44.937 0±0.594 0), the difference was statistically significant (P<0.05). The absorbance A value in the pretreated 2 d group was 41.698 0±0.724 0, which was significantly higher than the pretreatment 4 d group (absorbance A value 34.049 0±0.323 0) and 6 d group (absorbance A value 33.524 0±0.639 0). The difference was statistically significant (P<0.05). There was no significant difference in the expression of the pretreated 4 d group compared with the 6 d group (P=0.068). Conclusions Hyperbaric oxygen preconditioning can significantly promote the survival of rat flaps after transplantation. Preoperative hyperbaric oxygen preconditioning once daily for 4 consecutive days, can enhance the tolerance of flap tissue to ischemia and anoxia and reduce tissue ischemia-reperfusion injury.