A measurement method of diffraction efficiency of dot-matrix holography was proposed and an optical system was built. The first order (positive and negative) diffractive beams were separated from the zero and second ones skillfully by placing a plane reflect mirror within the system optical path to make a photodetector receive the first order (positive and negative) diffractive beams only. The direction variation of interference fringe cannot affect the experiment value. A half-wave plate was placed behind the laser and the power of the reflected beams and diffractive beams were received by two same photodetectors. At last the diffraction efficiency of eight different holographic security identifiers were calculated.