Objective To systematically evaluate the risk factors of postoperative delirium (POD) after knee arthroplasty (KA). Methods Systematic review was performed using literature concerning the risk factors of POD in patients undergoing KA from Chinese and English databases including CNKI, Wanfang, VIP, CBM, PubMed, the Cochrane Library, Web of Science, CINAHL, Embase, and EBSCO. The retrieval time was from the establishment of the databases to June 2020. The literature was screened by two researchers according to the principle of PICOS, and the quality of the included literature was evaluated according to the tools of JBI evidence‑based medicine. The data were analyzed by Stata 15.0 software. Results Finally, 9 descriptive studies, 3 cohort studies, and 1 randomized controlled trial (RCT) were included with a total of 1 195 837 patients. The results of Meta‑analysis showed that the risk factors of POD in patients with KA were as follows: compared with 70 years old, age>80 years old {odds ratio (OR) 31.22 [95% confidence interval (CI) 10.58, 92.14], P<0.001} and 70‒80 years old [OR 3.98 (95%CI 3.00, 5.28), P<0.001], compared with<65 years old, age≥65 years old [OR 1.12 (95%CI 1.08, 1.16), P<0.001], complicated with renal diseases [OR 2.07 (95%CI 1.56, 2.76), P<0.001], diabetes [OR 1.91 (95%CI 1.64, 2.22), P<0.001], dementia [OR 6.00 (95%CI 1.72, 20.90), P=0.005], history of mental illness [OR 3.90 (95%CI 2.83, 5.38), P<0.001] and anxiety and depression [OR 1.53 (95%CI 1.31, 1.78), P<0.001], electrolyte disorder [OR 2.26 (95%CI 1.37, 3.75), P=0.002], hypoxemia [OR 2.16 (95%CI 1.05, 4.41), P=0.035], postoperative benzodiazepine drugs [OR 2.29 (95%CI 2.09, 2.52), P<0.001], postoperative and long‑term administration of opioids [OR 1.25 (95%CI 1.09, 1.44), P=0.002], [OR 2.98 (95%CI 1.38, 6.41), P=0.005]. It cannot be considered hypo‑hemoglobin [OR 2.37 (95%CI 0.67, 8.45), P=0.183] as the risk factors of POD in patients with KA. Conclusions KA Patients have higher risk of POD with advanced age, basic metabolic diseases such as renal diseases and diabetes, dementia, history of mental illness, anxiety and depression, electrolyte disturbance, hypoxemia, postoperative benzodiazepine use, postoperative and long‑term use of opioids. Low‑hemoglobin cannot be considered as a risk factor.