A large piece of dandruff on the scalp may be caused by head simple chaffing, seborrheic dermatitis, psoriasis and other diseases. 1. Head simple chaff rash: this disease may be caused by malassezia infection, staphylococcus epidermidis and other pathogenic bacteria infection, but also with sebaceous gland secretion, heredity, bad hygiene habits and other factors. The main symptom is the appearance of powdery dry or chaff-like scales on the scalp, which can be manifested as a large piece of dandruff on the scalp, often accompanied by itching. 2. Seborrheic dermatitis: the cause of this disease is not clear, but may be related to heredity, pathogenic bacterial infections, immune dysfunction, neurological dysfunction and other reasons, often triggered by fatigue, emotional stress and other factors. If the lesions occur in the scalp, non-inflammatory patients will appear chaff-like flaking. 3. Psoriasis: research has shown that the disease occurs with heredity, autoimmune dysfunction, environmental factors and other reasons, common symptoms have thick scales on the scalp, so patients will have a large piece of scalp dandruff performance. There may be other reasons that can cause a large piece of dandruff on the scalp, it is recommended to go to the hospital in a timely manner to improve the relevant examination to clarify the specific causes.