A diffraction-based opto-electronic system is developed for precise measurement of entrance slit width of solar irradiance radiometer. The system use laser as light source, a NMOS linear array as opto-electronic sensor, a mixed-signal MCU C8051F as host controller, a CPLD as timing generator and on-chip 12-bit ADC. In addition, to achieve high-precision measurement, it uses least-squares method fitting the measured data to extract strip center location. Slit width can be measured within the range from 0.02 mm to 0.5 mm, in the measurement of entrance width of solar irradiance radiometer whose nominal width is 0.1 mm, the total uncertainty is 2.5×10－3 μm.