The cause of dandruff after washing your hair may be caused by fungal overgrowth on the scalp, allergy to toiletries and psoriasis. 1. Fungal overpopulation: Malassezia is a kind of fungus that grows on the human scalp, when it overpropagates, it will cause abnormal oil secretion of the scalp, which will lead to an increase in dandruff. 2. Allergy of washing and care products: the use of washing and care products containing polydimethylsiloxane, sodium chloride and other ingredients may cause scalp allergy, or excessive dehydration of the scalp, resulting in increased dandruff. 3. Psoriasis: related to immune disorders, if it occurs in the head, it can lead to hyperkeratosis of the scalp with keratosis pilaris, resulting in a large number of silvery-white flakes. If the phenomenon of increased dandruff persists without relief, it is recommended to go to the hospital for treatment after clarifying the cause of the disease.