Itchy scalp after washing hair may be caused by contact dermatitis, or seborrheic dermatitis, psoriasis and other diseases. 1. Contact dermatitis: If the shampoo and hair film used contain sensitizers, they are likely to cause an allergic reaction, which will lead to an itchy scalp after washing the hair. 2. Seborrheic dermatitis: the cause of the disease is unknown, may be related to genetics, infections, etc., and is most common in the head, face, armpits and chest and other parts of the body. If you suffer from seborrheic dermatitis of the scalp, the scalp will appear greasy dandruff, accompanied by itching, can appear after washing hair scalp itching. 3. Psoriasis: This disease is an immune skin disease, which can lead to the appearance of red patches on the affected skin, covered with white scales, with itching, occurring in the scalp, limbs, trunk and other parts of the scalp, scalp itchiness after washing hair can occur. Itchy scalp after washing hair, may also be caused by other reasons, it is recommended that patients go to the regular hospital in a timely manner, after a clear diagnosis, to give targeted treatment.