Background Blood eosinophil levels are a known marker for the effects of therapy in patients with chronic obstructive pulmonary disease (COPD). This study aimed to clarify the cutoff values for blood eosinophils (EOS) to predict exacerbation risk and prognosis of acute exacerbation COPD (AECOPD) and investigate their correlation using inflammatory indicators and clinical characteristics. Methods In this observational study of 174 patients with AECOPD, we assessed the relationship between EOS and COPD. According to the percentage of blood EOS, patients were grouped into two groups (Group 1: EOS <2%, n=98; Group 2: EOS ≥2%, n=76), and Group 2 was further divided into Group A (2%≤ EOS <4%) and Group B (EOS ≥4%) based on a cutoff value of 4%. Patients received standardized treatment after collection of peripheral blood specimen. Associations of EOS with laboratory indicators before any treatment in hospital and with clinical data were compared. Results Patients in Group 1 showed significantly severe inflammation, worse pulmonary function, longer length of stay (LOS) (P<0.001), higher mMRC score (P<0.05), higher CAT score (P<0.05), higher rates of mortality (P<0.05), and greater noninvasive mechanical ventilation usage (P<0.05) compared with Group 2. Intriguingly, the CRP, total mMRC and CAT scores of patients in Group A were significantly lower than those in Group B (P<0.001; P<0.01; P<0.05, respectively). Pearson correlation analysis showed that a low percentage blood eosinophil level was negatively associated with higher WBC count (r=–0.155, P<0.05), NLR (r=–0.227, P<0.01) and CRP (r=–0.308, P<0.01). Conclusions Different cutoff values for percentage blood EOS might be useful biomarkers for predicting the severity of exacerbation and prognosis of inpatients with AECOPD.