Objective    To investigate the effect of delayed surgery with different length and various degrees of ischemia on flap survival.    Methods    125 adult ICR mice were randomly divided into 5 groups. In the control group, flaps based on iliolumbar vessels with a size of 4.5 cm×1.5 cm were established. The four delayed groups were divided into vessels-preserved 1- and 2-week delay groups and vessels-transected 1-and 2-week delay groups. After the delay, the same flap model as the control group was established. Then, the necrosis rate of the flap was calculated, the arterial diameter and blood perfusion of the flap were measured by a laser speckle imaging device, and the blood vessels in the flap were marked by a CD31 antibody. The data of each group were statistically compared by one-way ANOVA.    Results    7 days after operation, the flap necrosis rate in each delayed group was lower than that in the control group (P<0.001). The diameter and perfusion intensity of the pedicle and the choke area in the four delayed groups were significantly higher than those in the control group (P<0.05). The survival rate of the flap, the diameter of the pedicle and choke arteries, and the blood perfusion intensity in the two vessels-transected groups were significantly higher than those in the two vessels-preserved groups (P<0.05); There was no significant difference in the various parameters between the 1-week delay group and the 2-week delay group (P>0.05). At 0 days after surgery, the vascular density in the four delayed groups was significantly higher than that in the control group (P<0.001), but there was no significant difference at 7 days after surgery (P=0.883).     Conclusion    The best efficacy can be achieved by delaying only one week. In addition, during the delay period, the flap should be in the state of the most serious ischemia without tissue necrosis.