Chronic obstructive pulmonary disease (COPD) is characterized by irreversible airflow limitation, and hyperinflation often results in diaphragm dysfunction. Diaphragm is the most important respiratory muscle, and its functional impairment will significantly affect the prognosis of patients. Ultrasound has been used to observe the shape and movement of the diaphragm because of its noninvasion, convenience and good repeatability. The application of ultrasound technique in COPD has been further expanded with the continuous progress of the theory and technology of ultrasound. This paper reviews the research progress of diaphragmatic ultrasound in patients with COPD, including its theoretical basis, evaluation indexes, examination methods, and clinical applications.