The correlation between body mass index (BMI) and the outcomes for patients receiving surgical intervention for abdominal aortic aneurysm (AAA) is still debated. Consequently, this study seeks to investigate the association between BMI and mortality following elective open abdominal aortic aneurysm repair (EOAR). This study involves a secondary analysis derived from data collected in a prospective cohort framework. It focuses on individuals who received EOAR across ten vascular surgery centers in Denmark from January 1, 2000, to December 31, 2014. The analysis employed a Cox proportional hazards regression model to examine the connection between BMI and mortality following EOAR. To detect any potential nonlinear associations, Cox regression with cubic spline functions and smooth curve fitting was utilized. Furthermore, we performed numerous sensitivity and subgroup evaluations to ensure the reliability of our results. Upon controlling confounding variables, the analysis demonstrated a clear inverse correlation between BMI and mortality following EOAR. Specifically, each 1 kg/m² increase in BMI corresponded to a 2.7% reduction in mortality risk (HR = 0.973; 95% CI: 0.958-0.988). Compared to individuals with normal weight, those classified as underweight faced a 62.6% higher risk of mortality (HR = 1.626; 95% CI: 1.142-2.314), whereas overweight and obese individuals experienced a reduction in mortality risk by 18.2% (HR = 0.818; 95% CI: 0.722-0.928) and 15.8% (HR = 0.842; 95% CI: 0.721-0.998), respectively. Additionally, the study identified a nonlinear relationship between BMI and postoperative mortality, with a critical point at a BMI of 25.78 kg/m². Below this inflection point, each 1 kg/m² decrease in BMI significantly increased mortality risk by 8% (HR = 0.920; 95% CI: 0.893-0.949). Beyond this inflection point, further increases in BMI did not significantly impact mortality risk. In patients undergoing EOAR, an increase in BMI is significantly negatively associated with postoperative mortality. Additionally, a specific nonlinear relationship exists between BMI and postoperative mortality, with a BMI inflection point at 25.78 kg/m². Clinicians should carefully weigh surgical risks for patients with a BMI under 25.78 kg/m² and consider delaying surgery to enhance nutrition. In contrast, those overweight or obese may not require pre-surgery weight management.