Background: Malnutrition is common in critically ill patients, but nutrition status in critically ill patients with acute kidney injury (AKI) has been poorly studied. Our study aimed to investigate the relationship between malnutrition risk and the occurrence and prognosis of AKI in elderly patients in the intensive care unit (ICU). Methods: Data were extracted from the Beijing Acute Kidney Injury Trial (BAKIT). A total of 1873 elderly patients were included and compared according to the clinical characteristics of AKI and non-AKI groups, and those of survivors and non-survivors of AKI in this study. Receiver operating characteristic (ROC) curves were used to analyse the predictive value of the modified Nutrition Risk in Critically III (mNUTRIC) score for the occurrence and 28-day prognosis of AKI. Multivariate Cox regression analysis was used to evaluate the effect of the mNUTRIC score on the 28-day mortality in AKI patients. Results: Compared with the non-AKI group, AKI patients had higher mNUTRIC scores, and non-survivors had higher mNUTRIC scores than survivors in AKI population. Moreover, multivariate Cox regression showed that 28-day mortality in AKI patients increased by 9.8% (95% CI, 1.018-1.184) for every point increase in the mNUTRIC score, and the mNUTRIC score had good predictive ability for the occurrence of AKI and 28-day mortality in AKI patients. The mortality of AKI patients with mNUTRIC > 4 was significantly increased. Conclusions: The elderly patients are at high risk of malnutrition, which affects the occurrence and prognosis of AKI. Adequate attention should be given to the nutritional status of elderly patients.