Whether and how long it takes for a knee effusion to heal on its own depends on the nature and amount of the knee effusion and the causative factors. If the amount of knee effusion is less than 50 ml, it usually takes about 4 weeks to heal on its own, and care should be taken to reduce weight bearing and activities of the knee during this period. However, if the amount of knee effusion exceeds 50 ml and there is a positive floating patella test, it is less likely to be self-absorbed and self-healing is more difficult, and a sterile puncture is required to extract the effusion. In addition, the causative factors of knee effusion also need to be considered. If it originates from trauma or high activity, attention to braking the knee joint and reducing weight bearing can effectively improve it. However, if the effusion is caused by infectious inflammation of the knee joint, antibiotic treatment should be started immediately; most are unlikely to heal on their own and can easily recur and worsen. In the case of knee effusion, it is necessary to actively search for the causative factors and remove the cause, as well as to determine the amount of effusion.