Abstract: Blood eosinophil(EOS) is recommended as a biomarker of chronic obstructive pulmonary disease(COPD) in GOLD 2017 for the first time. GOLD 2020 emphasizes that blood EOS is of great benefits in guiding the use of inhaled corticosteroids(ICS) in patients with stable COPD who are suffering from the high risk of acute exacerbations. Blood EOS highlights personalized assessment and precise treatment when making clinical decisions, which has great potential to promote the treatment of COPD from "risk management" to "targeted". However, there are still many controversies in the efficacy and safety when it comes to clinical application. Firstly, whether blood EOS can truly reflect the severity of airway EOS inflammation and disease and the risk of acute exacerbations in COPD patients. Secondly, whether the curative effect of adding ICS in patients with elevated EOS for preventing exacerbations is accurate. Thirdly, the benefit-risk ratio of the curative effect and the side effect such as pneumonia of ICS should be taken into consider in patients with elevated EOS. Fourthly, the threshold used to predict the risk of acute exacerbations and the curative effect of ICS isn't yet unified. Finally, the curative effect of targeted treatment therapy associated with blood EOS is uneven. Recently, blood EOS gradually becomes a hotspot and a focus of controversies in the field of treatment for COPD patients, with related researches emerging in an endless stream. This review focuses on EOS, including the progress and controversy about the stability, the efficacy to forecast the airway inflammation, disease severity and the risk of the acute exacerbations, the value in guiding the clinical application of ICS and targeted therapy, in order to have a deeper understanding in the efficacy and safety of EOS.