Three and a half year old baby boy is always nosebleed, common causes are: dry nasal mucosa, allergic rhinitis, platelet abnormalities. 1. dry nasal mucosa: if the baby is often in a dry environment, it is easy to lead to dry nasal mucosa, if the baby is always picking the nose, it is easy to cause the rupture of the capillaries in the nasal cavity, the appearance of the old nosebleed. 2. Allergic rhinitis: often the nose itchy performance, which leads to the child often picking the nose, resulting in nasal mucosal erosion, always nosebleed situation. And allergic rhinitis will also make the capillary expansion and congestion, due to the high permeability of capillaries, so there will also be frequent nosebleeds. 3. Platelet abnormality: if the child’s platelets are significantly lower than normal, resulting in always nosebleeds, be alert to leukemia, thrombocytopenic purpura and other diseases. In short, there are many causes of nosebleeds in three and a half year old male babies, it is recommended to go to the regular hospital in time.