Delirium is an acutely occurred neurocognitive disorder characterized by fluctuating symptoms of inattention, altered consciousness and cognitive dysfunction. Delirium is reported to occur in 4% to 65% of postoperative patients depending on the population, and is especially common in older patients. Postoperative delirium is disturbing to patients and their families, and it is a strong predictor of both early and long-term worse outcomes including increased non-delirium complications, increased perioperative mortality, shortened overall survival, declined cognitive function, and lowered quality of life. Although ketamine/esketamine has anti-inflammatory and neuroprotective effects, evidence on its efficacy in reducing postoperative delirium remains inconsistent and inconclusive. Existing studies are limited by heterogeneity, small sample sizes, single-center designs, and a focus on specific type of surgery. Research on elderly high-risk patients is lacking, and most studies administer the drug intraoperatively, with limited exploration of postoperative use. The optimal dosing and timing for POD prevention are unclear. This study aims to carry out a multicenter, single-blind, placebo-controlled, large-sample randomized controlled trial assessing the effect of low-dose esketamine, given intraoperatively and postoperatively, on delirium in elderly high-risk patients undergoing major non-cardiac surgery.