Raining knee pain may be caused by rheumatoid arthritis, rheumatoid arthritis, knee synovitis and degenerative arthritis. 1. Rheumatoid arthritis: the cause of rheumatoid arthritis is currently in the exploratory stage, and is mostly considered to be related to heredity, autoimmunity and other factors. Patients will experience joint pain, swelling, joint deformity and other symptoms. In addition, due to the rainy days will make the blood circulation of the knee joints slow down, the patient will appear knee pain aggravation performance. 2. Rheumatoid arthritis: Rheumatoid arthritis is considered to be caused by group A streptococcal infection, and common triggers include cold and humidity. Patients will experience redness, swelling, pain and burning sensation in large joints, so patients will experience knee pain on rainy days. 3. Knee synovitis: the cause of knee synovitis is not clear, and it is considered to be caused by knee injury and degenerative changes of the knee joint. Patients will show knee swelling, pain, limited activities, etc. Patients may have symptoms of increased knee pain on rainy days. 4. Other: knee pain on a rainy day is also considered to be degenerative arthritis, knee fracture, osteophytes, old knee trauma and other conditions. With the symptoms of knee pain on rainy days, it is recommended to go to the hospital in time to seek medical treatment and choose the appropriate treatment measures under the guidance of the doctor.