The most common cause of tear duct obstruction in infants is an obstruction of the Hasner valve at the lower end of the nasolacrimal duct. There is a half-moon shaped valve at the lower opening of the nasolacrimal duct, which is closed before birth and gradually opens after birth to act as a one-way valve. If the valve is not opened after a few weeks of life, the infant may experience tear accumulation or tear overflow, increased eye discharge and other symptoms of lacrimal obstruction. After a few days of massage, if the symptoms are relieved, the Hasner valve is opened; if the symptoms are not relieved, a lacrimal tract exploration is needed, and if the lacrimal tract exploration is successful, most of the infant’s lacrimal tract obstruction problems can be solved. There are also some rare causes of infant lacrimal duct obstruction, such as congenital atresia, congenital tear dots, lacrimal sac tumors, nasal and sinus tumors, etc. These cases cannot be solved by massage or exploration or cannot be explored, so further examination is needed to clarify the diagnosis and perform symptomatic treatment such as lacrimal duct reconstruction, tear dots reconstruction and tumor removal. In conclusion, the most likely cause of tear duct obstruction in infants is Hasner valve obstruction, which can be treated by massage or probing, supplemented by antibiotic drops. If the result is not good, further examination should be done to investigate some rare causes.