A piece of dandruff on the scalp may be suffering from seborrheic dermatitis, scalp psoriasis, tinea capitis and other diseases. 1. Seborrheic dermatitis: the disease may be related to Malassezia colonization, increased lipids, impaired skin barrier function, immune response and individual susceptibility, which may be manifested as a patch of greasy scales on the scalp, accompanied by erythema, itching and other symptoms. 2. Scalp psoriasis: it is the result of the joint action of genetic factors, immunity factors and environmental factors, and is often manifested as red patches with thick scales on the scalp, accompanied by itching. 3. Tinea capitis: caused by fungal infection of the scalp and hair, there will also be a piece of dandruff on the scalp, accompanied by itching and hair loss symptoms. There are many reasons for dandruff on the scalp, and it is recommended to go to the dermatologist in time to clarify the cause of the disease and then use reasonable medication under the guidance of the doctor.