1. A method, allowing a 2D character to perform a 3D motion, comprising: a) corresponding 3D human motion data to a virtual bone, and recording all motion data by a matrix to obtain a motion bone world matrix; b) setting up a virtual camera for observing the 3D motion and the virtual bone to define a view point for observing the 3D motion, the view point being any position in a 3D world; calculating a motion angle to obtain rotation and angle information of human body parts when performing the 3D motion to further obtain a virtual camera transformation matrix calculated according to the virtual camera as an observation view point; and transforming 3D coordinates to 2D coordinates and a depth to obtain coordinates and depth information of the 3D motion and the body parts in motion; c) executing a sprite angle calculation to determine which image of which image of which angle is to be displayed when presenting the 3D motion by a sprite, determining a corresponding relationship between an angle precision and all images of a sprite image set, executing a 2D rotation calculation step, executing a 2D scale calculation and free-form transformation step, and executing a depth relation conversion calculation step to provide correct depth relations for all the joint points and the bones of the body parts when performing 2D animation, wherein a distance of every two of the joints of the body parts is calculated according to the position of the virtual camera and positions of the joints in the virtual camera view so as to obtain a vector representing the depth relation; every two joints are compared to determine a sequence between the two joints according to dot matrix vector multiplication; and d) processing, outputting and displaying a result.