Yellowish nasal mucus on the second day of a cold is usually caused by a viral infection of the upper respiratory tract secondary to a bacterial infection. Colds are usually caused by simple viral infections, and the phenomenon of yellowing of the nose on the second day is a common manifestation of secondary bacterial infections. When there is a secondary bacterial infection in the upper respiratory tract, white blood cells in the body, such as phagocytes, will gather at the site of inflammation and engulf the foreign bacteria, and then the bacterial-containing phagocytes will necrose and lyse. When the necrotic phagocytes, bacteria, necrotic mucosal debris and inflammatory exudate mix together, yellow nasal mucus is formed. When yellow nasal mucus occurs, antibiotics such as amoxicillin and cephradine can be applied under doctor’s guidance for treatment, and attention should be paid to drinking more water, eating more fresh fruits and vegetables, and paying attention to rest.