In a method for regulating a mechanical arm using a control computer. The method obtains a first position and a second position of a center of an image plane of an image capturing device on the mechanical arm by controlling movements of the mechanical arm, and calculates a movement vector from the first position to the second position. The method further calculates a regulating angle according to the movement vector and an axial vector of the mechanical arm, and moves the mechanical arm according to the regulating angle such that the axial vector is perpendicular to a measurement plane determined by the first object and the second object.