Chronic obstructive pulmonary disease (COPD) and asthma are common airway diseases, and there are similarities and differences between them. Serum eosinophil (EOS) has potential application value in the diagnosis, treatment, and prognosis of COPD and asthma. However, the clinical application value of serum EOS in these two diseases is controversial. In this work, recent research progress on the application of serum EOS in COPD and asthma was analyzed, and the potential application of serum EOS in the two diseases was expounded from several aspects, including the correlation between the pathogenesis of COPD and asthma and EOS, as well as the correlation between the diagnosis, treatment, and prognosis of the two diseases and serum EOS. The results revealed that serum EOS was important in the pathogenesis, diagnosis, treatment, and prognosis of COPD and asthma and exhibited a potential clinical application value. However, further study was needed to evaluate the threshold, which provided guidance for the clinical diagnosis and treatment of COPD and asthma.