Objective We explored the prognostic significance of controlling nutritional status (CONUT) score in older adults with heart failure with preserved ejection fraction (HFpEF) and compared CONUT with other objective nutritional indices. Methods This is a single-center retrospective cohort study in older adult coronary artery disease patients undergoing HFpEF. Clinical data and laboratory results were collected before discharge. CONUT, geriatric nutritional risk index (GNRI), and prognostic nutritional index (PNI) were calculated according to the formula. The primary endpoint of this study was readmission due to heart failure and all-cause mortality in the first year after hospitalization. Results A total of 371 older adults were enrolled. All patients were discharged and followed up for 1 year, and readmission for heart failure was 26% while all-cause mortality was 20%. Compared with the none and mild malnutrition risk group, the readmission rate for heart failure (HF) within 1 year (36% vs. 18%, 23%) and all-cause mortality rate in the moderate and severe malnutrition risk group (40% vs. 8%, 0%) were higher (P < 0.05). On multivariate logistic analysis, CONUT was not associated with readmission due to HF within 1 year. CONUT was significantly associated with all-cause mortality independently of GNRI or PNI, after adjustment for major confounders including age, bedridden; length of stay; history of chronic kidney disease; loop diuretics use; angiotensin-converting enzyme inhibitor/angiotensin receptor blocker and beta-adrenergic blocking agents use; New York Heart Association (NYHA) functional class; hemoglobin; potassium; Creatinine; triglycerides; glycosylated hemoglobin; brain natriuretic peptide; left ventricular ejection fraction; GNRI and PNI via multivariable Cox analysis (HR (95% CI) 1.764 (1.503, 2.071); 1.646 (1.359, 1.992); 1.764 (1.503, 2.071), respectively). Kaplan-Meier analysis revealed that the risk of all-cause mortality significantly increased in accordance with a higher CONUT (CONUT 5-12 compare to 0-1:HR (95% CI) 6.16 (3.78, 10.06); CONUT 2-4 compare to 0-1:HR (95% CI) 0.16 (0.10, 0.26)). CONUT showed the best area under the curve value (0.789) for the prediction of all-cause mortality compared with the other objective nutritional indices. Conclusion CONUT is a simple and strong prognostic indicator for the prediction of all-cause mortality in older adults with HFpEF.