Abstract: At the top of the target rod a mirror is mounted to be in the same plane asthat of target surface. In the meanwhile, above the beam pipe a laser device isinstalled so that the emitted laser beam goes along the certain line paralleled tothe center line of the beam pipe and exactly hits on the mirror and then makesdeflection. Therefore, measuring the angle between the incident beam and targetsurface is converted into measuring the angle between the laser incident beam andits deflected beam. Obviously, the measured angle is twice as big as the real anglebetween the incident beam and the target surface.