OBJECTIVE To investigate the incidence rate and risk factors for grade III flat anterior chamber (FAC) after glaucoma filtration surgery based on 5-year data. METHODS Patients who underwent glaucoma filtration surgery in Daping hospital from January 2009 to December 2013 were enrolled in this retrospective study. The incidence of grade III FAC following glaucoma filtration surgery was determined. Regression analysis was performed to investigate the influence of glaucoma type, surgical approach, age, and preoperative intraocular pressure (IOP) on the risk of postoperative FAC. RESULTS A total of 2179 eyes receiving anti-glaucoma surgery were included. Ninety-one eyes suffered from postoperative FAC, with an overall incidence rate of 4.18%. Of 471 eyes with primary open angle glaucoma (POAG), grade III FAC occurred in only 3 eyes (0.64%). Primary angle-closure glaucoma (PACG) was diagnosed in 1076 eyes, 39 (3.62%) of which developed grade III FAC, including 12 eyes (12/300, 4%) with acute PACG (aPACG) and 27 eyes (27/776, 3.48%) with chronic PACG (cPACG). Six of 259 eyes (2.32%) with secondary glaucoma, 28 of 186 eyes (15.05%) with neovasular glaucoma, 1 of 66 eyes (1.52%) with congenital glaucoma, and 14 of 115 eyes (12.17%) with remnant glaucoma suffered from grade III FAC. Of 6 eyes with mixed glaucoma, none developed grade III FAC after surgery. When stratified by surgical approach, 24 of 766 eyes (3.13%) undergoing trabeculectomy, 21 of 924 eyes (2.27%) treated by trabeculectomy plus mitomycin C (MMC), 18 of 109 eyes (16.51%) undergoing Ahmed glaucoma valve implantation, 23 of 201 eyes (11.44%) managed by Ahmed implantation plus MMC, and 5 of 133 eyes (3.76%) treated by Ahmed implantation plus lens extraction or vitrectomy developed grade III FAC. Logistic regression analysis revealed that factors including neovasular glaucoma, remnant glaucoma, glaucoma valve implantation, glaucoma valve implantation+MMC, glaucoma valve implantation+vitrectomy, age>60y, and IOP at admission >50 mm Hg were significantly associated with an increased risk for grade III FAC. CONCLUSIONS The overall incidence of grade III FAC after glaucoma filtration surgery is 4.18%. Patients with neovasular glaucoma and remnant glaucoma are at a higher risk of developing FAC. Ahmed glaucoma valve implantation is associated with a higher risk for grade III FAC compared with trabeculectomy. No significant correlation was observed between the use of MMC in glaucoma filtration surgery and the risk of postoperative FAC. Higher IOP at admission (>50 mm Hg) and old age (>60y) are risk factors for grade III FAC.