Aiming at solving the problem of joint angle measurement during the movement of small robots or robotic arms that cannot be equipped with angle encoders, this paper provides a small, lightweight and detachable measurement system. Compared with traditional magnetic and optical joint angle sensors, the principle of this method has the advantages of flexible installation and non-invasive measurement. By simply binding two inertial measurement units (IMUs) on both sides of the robot joint in any pose, real-time and high-precision robot joint angle measurement can be achieved through wireless transmission. At the same time, the relationship between the IMU and the joint pose changes dynamically with the robot, which can avoid measurement errors due to loose binding or robot vibration. In this paper, experiments were carried out on a knee joint rehabilitation training robot which can realize two rehabilitation actions, and the precise and stable joint angle curve of the robot is obtained.