A 54-year-old woman with an epiretinal membrane in her left eye accompanied by a shallow anterior chamber due to primary angle closure glaucoma underwent vitrectomy and cataract surgery. During the cataract surgery, immediately after the ultrasonic tip had been removed from the anterior chamber, anterior chamber flattening occurred. An intraoperative fundus examination showed the development of acute intraoperative choroidal effusion. Postoperatively, the anterior chamber remained shallow even after the choroidal detachment had subsided; capsular bag distension seen by ultrasound biomicroscopy suggested the development of early postoperative capsular block syndrome. After neodymium:yttrium-aluminium-garnet laser capsulotomy, the anterior chamber deepened. Depending on the perioperative period, the mechanism of a flat anterior chamber can change, and understanding the underlying mechanisms is required for appropriate treatment.