A joint bending state determining device , adapted to determine a bending state of a joint (J) such as a knee, an elbow, a hip joint, etc., comprising: a sensor group comprising: a first gravity sensor configured to generate a first polar angle and a first azimuth angle; and a second gravity sensor configured to generate a second polar angle and a second azimuth angle, wherein the first and second gravity sensors are adapted to be disposed on limps of a human body connected by said joint (J); and a processor connected with the sensor group , and configured to obtain a first length corresponding to the first gravity sensor and a second length corresponding to the second gravity sensor , to obtain a set of coordinates of a joint in a coordinate system according to the first polar angle, the first azimuth angle and the first length, to obtain a set of coordinates of the second gravity sensor in the coordinate system according to the set of coordinates of the joint, the second polar angle, the second azimuth angle and the second length, and to obtain a joint angle according to the set of coordinates of the second gravity sensor in the coordinate system, the first length and the second length; wherein the first length is associated with a distance between the first gravity sensor and the set of coordinates of the joint, and the second length is associated with a distance between the second gravity sensor and the set of coordinates of the joint; wherein the joint bending state determining device is adapted to determine which one of the first gravity sensor and the second gravity sensor has a higher charge level and to set the gravity sensor with the higher charge level to collect information of the first polar angle, the first azimuth angle, the second polar angle and the second azimuth angle, and to transmit the information to the processor . a first gravity sensor configured to generate a first polar angle and a first azimuth angle; and a second gravity sensor configured to generate a second polar angle and a second azimuth angle, wherein the first and second gravity sensors are adapted to be disposed on limps of a human body connected by said joint (J); and a processor connected with the sensor group , and configured to obtain a first length corresponding to the first gravity sensor and a second length corresponding to the second gravity sensor , to obtain a set of coordinates of a joint in a coordinate system according to the first polar angle, the first azimuth angle and the first length, to obtain a set of coordinates of the second gravity sensor in the coordinate system according to the set of coordinates of the joint, the second polar angle, the second azimuth angle and the second length, and to obtain a joint angle according to the set of coordinates of the second gravity sensor in the coordinate system, the first length and the second length; wherein the first length is associated with a distance between the first gravity sensor and the set of coordinates of the joint, and the second length is associated with a distance between the second gravity sensor and the set of coordinates of the joint; wherein the joint bending state determining device is adapted to determine which one of the first gravity sensor and the second gravity sensor has a higher charge level and to set the gravity sensor with the higher charge level to collect information of the first polar angle, the first azimuth angle, the second polar angle and the second azimuth angle, and to transmit the information to the processor . <EOS>