3-year-old baby double knee joint extension may be physiological phenomenon, may also be juvenile idiopathic arthritis, joint trauma, etc. Need to consult a doctor to clarify the cause of the disease after targeted treatment, including drugs and general treatment. The fetus is flexed in the uterus, and after birth, the tension of the flexor muscles is a little stronger than that of the extensor muscles, so the legs are bent. Coupled with the fact that human growth and development follows a top-to-bottom pattern, the muscles and bones of the lower limbs develop later. With age, the bones and muscles develop gradually, and the tension of the flexor and extensor muscles is gradually balanced. The baby’s knee hyperextension can then be gradually improved. If the baby’s knee joints are not straight and accompanied by fever and rash, it may be caused by juvenile idiopathic arthritis, which leads to swelling of the knee joints and limitation of activities, and repeated attacks for several years may lead to joint deformity. It is necessary to go to the hospital in time to complete the relevant examinations, and apply non-steroidal anti-inflammatory drugs (for example, ibuprofen, etc.), immunosuppressive drugs (for example, methotrexate, etc.) for comprehensive treatment. After trauma to the knee joint, the baby may not be able to straighten the joint due to fluid accumulation and blood swelling, and may need to be pierced or immobilized for rehabilitation. For severe cases, it is recommended to seek medical advice to clarify the cause of the disease and provide targeted treatment. All the above medications should be taken under medical supervision.