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Objective: To explore age-adapted left ventricular ejection fraction (LVEF) diagnostic thresholds for heart failure with preserved ejection fraction (HFpEF) in children. Methods: In this multicenter retrospective cohort study, children with chronic heart failure were selected from 3 546 hospitalized pediatric heart failure patients across 30 medical centers in 20 provinces, autonomous regions and municipalities in China from January 2013 to December 2022. Children were divided into 3 groups based on LVEF:<50%, 50%-<55%, and ≥55%. Age-stratified analyses were performed using cutoffs at 9, 10, 11, 12, and 13 years of age. Intergroup comparisons were conducted using the chi-square test or Fisher's exact test, Wilcoxon rank-sum test or Kruskal-Wallis test. Multivariable linear regression was used to analyze the independent association between LVEF categories and cardiac structural parameters. Results: A total of 1 018 pediatric patients were included in the analysis. Among them, 527 (51.8%) were male and 491 (48.2%) were female, with an age of 1.2 (0.4, 7.1) years. There were 447 children in LVEF <50% group, 62 children in 50%-<55% group, and 509 children in ≥55% group. Statistical differences in baseline clinical characteristics were observed among the 3 groups, including age, heart rate, respiratory symptoms, systemic congestion symptoms, congenital heart disease, cardiomyopathy cardiovascular events, growth retardation (all P<0.05). Age-stratified analysis showed that when using 9, 10 and 11 years of age as cutoffs, left ventricular end-systolic diameter (LVESD) differed between the LVEF <50% and 50%-<55% groups in children aged at or above the respective cutoff (all P<0.05). When using 12 and 13 years of age as the cutoff, left atrial diameter differed between the LVEF 50%-<55% and ≥55% groups in children below 12 and 13 years of age (both P<0.05). After adjustment for covariates using multivariable linear regression, when using 10 and 11 years of age as the cut-offs, LVEF were correlated with LVESD in both children aged ≥10 years and those aged ≥11 years (β=11.62 and 10.94,95%CI 2.38-20.85 and 1.32-20.56, P=0.018 and 0.029). And when using 12 years of age as the cut-offs, LVEF was correlated with left atrial diameter in children aged <12 years of age (β=-1.82,95%CI -3.29--0.35,P=0.015). Conclusion: Age-related heterogeneity may exist in the LVEF thresholds for pediatric HFpEF, an LVEF threshold of 55% is recommended for children younger than 12 years, whereas 50% consistent with adult criteria, may be considered for those aged 12 years or older.