There are many reasons for knee popping when bending the knee, which may be related to physiological popping, meniscus injury, knee joint free body and so on. 1. Physiological popping: Under physiological condition, the knee joint will make sound when bending due to its physiological structure, and this situation is called physiological popping. Generally, there is no pain or discomfort in the knee joint, and there is no dysfunction. 2. Meniscus injury: the knee joint has been sprained or has a history of strenuous exercise and trauma. It can lead to joint lasso after meniscus injury, and the meniscus can make a ringing sound when the knee joint is moving. 3. Knee joint free body: synovial chondroma or damaged meniscus, etc. form joint free body, detached from the normal position of the joint, free in the joint. When the knee joint is bent, it will also make a popping sound. When the knee pops when bending, it is recommended to consult a doctor in time, follow the doctor’s instructions for further examination, clarify the cause with the help of the doctor, and carry out targeted treatment or therapy.