There are various causes of itching down to the knee, which may be related to the skin disease itself, external factors, and multi-system lesions. 1. Diseases of the skin itself: e.g. eczema, herpetic disease, urticaria, ringworm, etc., usually manifested as itching, accompanied by pimples, erythema and so on. 2. caused by external factors: such as contact dermatitis caused by contact with strong acid, alkali, pollen, lint, metal, etc., scabies, lice and other parasites caused by skin diseases can also appear intense itching with erythema, papules and other changes, mosquito bites knee down to the skin can also appear itchy. 3. Multi-systemic lesions: cirrhosis, bile duct tumors and other liver and gallbladder diseases, uremia and other renal diseases, thyroid, diabetes and other endocrine diseases, malignant tumors and so on may cause itchy skin. It is recommended to go to the hospital in time after the appearance of the above symptoms, improve the examination, clarify the specific causes, and comply with the rules of medical treatment.