A joint bending state determining device comprises a sensor group and a processor. The sensor group comprises first and second gravity sensors. The first/second gravity sensor generates a first/second polar angle and a first/second azimuth angle. The processor obtains a first length corresponding to the first gravity sensor and a second length corresponding to the second gravity sensor, obtains 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, obtains 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 obtains 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.