Heart failure (HF) is associated with poor prognosis. While various immune markers have been linked to HF outcomes, the relationship between eosinophils (EOS) and prognosis across different HF phenotypes remains unclear and controversial. In this study, we analyzed 2,677 HF patients from the Medical Information Mart for Intensive Care III (MIMIC-III) database, categorizing them into reduced ejection fraction (≤ 50%) and preserved ejection fraction (> 50%) groups. Multiple logistic regression, Kaplan-Meier survival curve estimation, Cox proportional hazards model, and restricted cubic splines were employed to assess the association between admission EOS and mortality. Subgroup analyses were conducted based on age, gender, sepsis, atrial fibrillation, hypertension and rheumatic and immunological diseases and mechanical ventilation. A total of 954 patients with heart failure with reduced ejection fraction (HFrEF) and 1723 patients with heart failure with preserved ejection fraction (HFpEF) were enrolled, with a female proportion of 37.84% and 56.88%, respectively. The median age of patients was 74.46 (IQR: 63.34-83.26) years and the median EOS was 0.09 (IQR: 0.04-0.20). Among these patients, in-hospital mortality was 14.5% (389 patients), and the overall 90-day mortality rate reached 27.12% (726 patients). There were no significant differences in EOS levels between the HFrEF and HFpEF groups (0.1 vs. 0.09, P = 0.154). After adjusting for confounders, multivariate logistic regression showed that higher EOS quartiles were associated with reduced in-hospital mortality in HFrEF patients, with quartile 3 (Q3) (OR 0.54, 95% CI 0.31-0.96, P = 0.035) and Q4 (OR 0.37, 95% CI 0.19-0.71, P = 0.003) showing significant protective effects compared with the lowest EOS quartile (Q1), while Q2 did not reach statistical significance (P = 0.931). Subgroup analyses yielded similar findings. Multivariate Cox regression demonstrated a reduced hazard ratio in Q4 (HR 0.64, 95% CI 0.44-0.94, P = 0.023) compared with Q1 in HFrEF patients. However, no significant association was observed between EOS and prognosis in HFpEF. Our findings indicated that higher EOS was associated with improved in-hospital and 90-day survival in ICU patients with HFrEF, but not in those with HFpEF.