Knee pain on rainy days may be caused by rheumatoid arthritis, scarring and environmental factors. 1. Rheumatoid arthritis: it is a kind of allergic reaction disease, which may be related to group A streptococcal infection, and can be induced by cold and wet weather. Patients are prone to increased nerve sensitivity of the joints as well as decreased temperature in the joint area, resulting in knee pain symptoms. 2. Scarring: If the knee joint area has been previously traumatized, scarring may form in the subcutaneous tissue after the skin has healed. Part of the scar may have nerve fiber clusters, which are sensitive to external climate change. When the knee area is stimulated by cold and dampness, it may lead to contraction of the scar tissue and the surrounding muscles and blood vessels, resulting in pain symptoms. 3. Environmental factors: if the patient usually do not pay attention to warmth, resulting in the knee for a long time by the cool wind stimulation, will lead to muscle and blood vessel contraction, thus causing pain symptoms. It is recommended that patients with knee pain on rainy days go to the hospital in a timely manner, and follow the doctor’s instructions for treatment to avoid delaying their condition.