Background Critically ill chronic obstructive pulmonary disease (COPD) patients admitted to an intensive care unit (ICU) due to respiratory failure are at particularly high risk of Aspergillus infection. The serum galactomannan index (GMI) has proven to be one of the prognostic criteria for invasive pulmonary aspergillosis (IPA) in classical immunocompromised patients. However, the prognostic value of serum GMI in critically ill COPD patients needs evaluation. The purpose of this study is to investigate the prognostic value of serum GMI in patients with severe COPD. Methods In this single-center prospective cohort study, serum samples for GMI assay were collected twice a week from the first day of ICU admission to the day of the patients' discharge or death. Patients were divided into two groups according to their clinical outcome on the 28th day of their ICU admission. Univariate analysis and survival analysis were tested in these two groups. Results One hundred and fifty-three critically ill COPD patients were included and were divided into survival group (106 cases) and non-survival group (47 cases) according to their outcome. Univariate analysis showed that the highest GMI level during the first week after admission (GMI-high 1st week) was statistically different between the two groups. Independent prognostic factors for poor outcome in severe COPD patients were: GMI-high 1st week >0.5 (RR: 4.04, 95% Cl: 2.17-7.51) combined with accumulative dosage of corticosteroids >216 mg before the RICU admission (RR: 2.25, 95% Cl: 1.11-4.56) and clearance of creatinine (Ccr) <= 64.31 ml/min (RR: 2.48, 95% Cl: 1.22-5.07). Conclusions The positive GMI-high 1st week (>0.5) combined with an accumulative dosage of corticosteroids >216 mg before the ICU admission and a low Ccr may predicate a poor outcome of critically ill COPD patients.