A method controls motions of a mechanical arm using an electronic device. The mechanical arm includes an x-axis, a y-axis, and a z-axis. The method sets a step angle of a motor installed in line with the z-axis, and predefines angles of rotation as reference points regarding orientation of the rotating arm when rotated. The method further calculates a rotation angle of the rotating arm in a polar coordinate system, calculates coordinate motions of the x-axis and the y-axis if a free end of the rotating arm orientates to one of the reference points, and calculates an offset correction value of a pole origin position of the polar coordinate system. After correcting each of the coordinate motions and the pole origin position, the method controls the mechanical arm to move, and drives the rotating arm to reach another reference point according to a multiple of the step angle.