To compare the efficacy and safety of intravitreal injections of ranibizumab (IVR) before and at the end of vitrectomy in proliferative diabetic retinopathy (PDR) patients.A prospective comparative study was performed on 60 eyes of 52 PDR patients who received ranibizumab injection (0.5 mg/0.05 ml) 3-5 days before vitrectomy (preoperative group) and 55 eyes of 50 PDR patients who received ranibizumab injection (0.5 mg/0.05 ml) at the end of vitrectomy (intraoperative group). Intra- and postoperative indices were collected for further comparison.Postoperative best-corrected visual acuity (BCVA) in preoperative group was better than in intraoperative group at 1 week after surgery (P < 0.05) but comparable at 1- and 3-month follow-up (P = 0.20 and P = 0.37, respectively). Central retinal thickness (CRT) in preoperative group was lower than in intraoperative group at 1 week postoperatively (P < 0.05), but comparable at 1- and 3-month follow-up (P = 0.39 and P = 0.77, respectively). The average surgery time was significantly shorter in preoperative group than in intraoperative group (61.50 ± 11.44 min vs. 74.49 ± 12.01 min, P < 0.01). The incidence of intraoperative bleeding was significant lower in preoperative group than in intraoperative group (21.7% vs. 40.0%, P < 0.05). Moreover, the incidence of intraocular electrocoagulation use, iatrogenic retinal breaks, relaxing retinotomy and silicone oil tamponade were all significantly lower in preoperative group than that in intraoperative group (P < 0.05, respectively). The incidences of postoperative vitreous hemorrhage (VH), neovascular glaucoma (NVG), recurrent retinal detachment, postoperative fibrovascular proliferation progression and reoperation showed no statistical differences between the two groups (P > 0.05, respectively). Both groups had no ocular or system adverse events during observation period.In vitrectomy for PDR, preoperative IVR can significantly reduce surgery time and lower the incidence of intraoperative bleeding, intraocular electrocoagulation use, iatrogenic retinal breaks, relaxing retinotomy and silicone oil tamponade during surgery and gain short-term better postoperative BCVA and thinner CRT.ClinicalTrials.gov (identifier, NCT05408416).© 2022. The Author(s).