The diaphragm, the most important respiratory muscle, is responsible for about 75% of the work of breathing. Diaphragmatic function can be assessed by measuring its thickness excursion by ultrasound, thereby assisting clinicians in making corresponding decisions. Diaphragmatic ultrasound has potential value in predicting the outcome of weaning in patients with mechanical ventilation, assisting the diagnosis of chronic obstructive pulmonary disease, predicting residual muscle relaxation after general anesthesia, predicting postoperative pulmonary complications, and assessing the effect of rehabilitation training. This article reviewed the ultrasound assessment methods of the diaphragm and its clinical application, thus providing guide for the clinicians to assess diaphragm function using ultrasound-related indicators and helping clinical work.