Postoperative delirium (POD) is highly prevalent in older patients with type 2 diabetes mellitus (T2DM). Cerebral insulin resistance is considered a key pathophysiological mechanism underlying this association. This study aims to investigate whether intranasal insulin (INI) can prevent POD and early postoperative cognitive decline by improving cerebral insulin resistance. We will conduct a randomized, double-blind, placebo-controlled multicenter trial. Older T2DM patients will be randomly assigned to receive intranasal insulin at doses of 40 IU or 80 IU, or placebo. The primary objective is to evaluate the efficacy of INI in preventing POD and to determine the dose-response relationship.