Malnutrition is a prevalent yet underrecognized comorbidity in coronary artery bypass grafting (CABG) patients. Modified BMI (mBMI), defined as albumin × BMI, has been proposed as a more accurate indicator of nutritional risk. This study examined the association between preoperative mBMI and in-hospital outcomes following Off-pump CABG (OPCABG).This retrospective cohort study included patients with coronary artery disease who underwent isolated OPCABG at eight cardiac centers. Patients were stratified into tertiles based on preoperative mBMI values. The primary endpoint was in-hospital mortality. Logistic regression and restricted cubic spline models were used to evaluate the association between mBMI and outcomes. Subgroup analyses were performed based on clinical characteristics including sex, age, renal function, and comorbidity burden.Among 6,667 patients, the lowest mBMI tertile had higher in-hospital mortality (2.52 % vs 1.53 % vs 1.17 %), cerebral infarction (2.48 % vs 2.34 % vs 1.39 %), and dialysis rates (1.04 % vs 0.54 % vs 0.36 %) compared to higher tertiles (all P < 0.05). Restricted cubic spline analysis revealed an inverse association between mBMI and the odds of in-hospital mortality, with the lowest risk observed around mBMI 1300-1350. Subgroup analyses demonstrated this relationship was particularly significant among females, patients with eGFR ≤ 90, and those with ASA score > 3 (P for association = 0.023, 0.013, and 0.049, respectively).Lower mBMI is associated with an increased risk of in-hospital mortality and multiple adverse outcomes in patients undergoing OPCABG, particularly among females and those with renal impairment or systemic comorbidities.Trial registration: Chinese Clinical Trial Registry: Chictr2400085741.© 2025 The Authors. Published by Elsevier B.V.