It is important to measure micron grade of the aperture size in the field of industrial production. Based on the Fraunhofer diffraction principle of circular hole，there is a circular diffraction pattern on the receiving screen when laser passes through a small hole，the laser fluency in the area of the airy disk changes with the aperture size. Photovoltaic cell as the photoelectric detector only receives the laser intensity signal at the center of airy disk but not the higher order diffraction rings. The voltage value transformed from the photovoltaic cell includes the aperture size. The detection system is built with single - chip microcomputer and dual optical path. After calibration test using known aperture size，the relation between the output voltage signal and aperture size can be acquired. The functional relation of the detection system can be obtained by linear fitting the experimental results.Experimental result shows that this system can test the aperture sizes varied from 50 to 300 μm.