Objective To investigate the timing of administration of Conbercept in the treatment of proliferative diabetic retinopathy (PDR) before operation and its effect on neovascularization vascular endothelial growth factor (VEGF).Methods 90 patients (90 eyes) with PDR who underwent vitrectomy (PPV) were randomly divided into groups A,B and C,with 30 cases in each group.Group A received no intravitreal injection of Conbercept before operation,group B received intravitreal injection of Conbercept 3 days before operation,and group C received intravitreal injection of Conbercept 5 days before operation.The operation condition,visual acuity level,concentration of vascular endothelial growth factor in aqueous humor and positive expression of vascular growth factor in neovascularization membrane were compared in the three groups.Results There was no significant difference in the incidence of iatrogenic hiatus during operation,transient high intraocular pressure and hyphema after operation in the three groups (P ＞ 0.05).Compared with group A,group B and C had shorter the operation time,less electrocoagulation,lower LogMAR BCVA (best corrected visual acuity) three months after operation (P ≤ 0.05).There was no significant difference in the concentration of vascular endothelial growth factor between group B and group C before vitreous injection (P ＞ 0.05).The expression of VEGF in aqueous humor of group B and group C was lower than that of group A (P ≤ 0.05),and the total positive expression rate of VEGF in neovascularization membrane of group B and group C was 76.67％ and 73.33％ respectively,lower than that of group A (100.0％,P ≤0.05).The vitreous re-bleeding of group B and group C was 6.67％ and 10.0％,lower than that of group A (32.14％,P ≤ 0.05).While there was no significant difference between group B and group C (P ＞0.05).Conclusions Vitreous injection of Conbercept before PPV in PDR patients can reduce the concentration of VEGF in aqueous humor and the positive expression rate of VEGF in neovascularization membrane,significantly improve visual acuity and reduce the incidence of postoperative complications.The effect of vitreous injection 3 and 5 days before PPV is basically the same.