Previous studies have demonstrated that the shock index (SI), age-adjusted shock index (ASI), and modified shock index (MSI) are useful for predicting massive transfusion (MT) and mortality in patients with traumatic injuries. However, studies have not been conducted on the use of the age-modified shock index (AMSI) to indicate the prognosis of patients with traumatic injuries. This study aimed to evaluate the predictive power of AMSI for MT and mortality. We hypothesized that AMSI would be superior to other indices in predicting outcomes in patients with traumatic injuries. This retrospective, single-center study was conducted at a level 1 trauma center and included consecutive patients who visited the trauma center between January 2016 and December 2022. The predictive value of AMSI for MT, in-hospital mortality, and 24 h mortality was assessed using receiver operating characteristic (ROC) analysis. We compared the area under the ROC curve (AUROC) of AMSI with those of SI, ASI, and MSI. In total, 6591 patients were included in the study, of whom 479 received MT. The in-hospital and 24 h mortality rates were 8.7% and 5.3%, respectively. The SI, ASI, MSI, and AMSI all showed better predictive performance for MT (AUC > 0.7) than that for in-hospital (AUC: 0.50, 0.61, 0.50, and 0.62) and 24 h mortality (AUC: 0.54, 0.56, 0.54, and 0.56). However, AMSI did not demonstrate superior performance compared with the other indices (SI, ASI, and MSI) in predicting both MT and 24 h mortality. AMSI demonstrated significantly better predictive performance for in-hospital mortality than the other indices; however, the difference from ASI was not substantial. This is likely because age has a significant impact on in-hospital mortality. Indices other than AMSI that are easier to compute may be more useful for the prognostic evaluation of patients with traumatic injuries.