Medial collateral ligament injury can be categorized into three degrees. I degree injury with pressure and pain at the medial collateral ligament alignment, but the sense of opening and negative lateral move test indicate mild injury of the medial collateral ligament with no joint instability. Lateral transfer test and open bacillus were positive in degree II injury, but there was a blocking sensation during lateral transfer, indicating incomplete injury of the medial collateral ligament, mild instability during knee flexion, and no instability during extension. Third-degree injuries are evident in the lateral carry test and the sense of opening, with no sense of blockage, indicating complete injury to the medial collateral ligament. It can be divided into two cases, if there is no posterior medial joint capsule injury, it is stable in extension and unstable in flexion position, and if there is concomitant posterior medial joint capsule injury, there is significant instability in both extension and flexion positions.