In order to improve the measuring accuracy of metal wire diameter, this work discusses the experimental scheme of laser diffraction method. Firstly, the theoretical formula for calculating a slit width is derived from the generation conditions of dark stripes in Fraunhofer single slit diffraction. According to Babinet theorem, The filaments and slits that are complementary obstacles produce the same diffraction patterns and therefore the metal wire diameter can be obtained by measuring the slit width.Then the experimental procedure for measuring the intensity distribution of metal wire diffraction is described in detail. Finally, based on the measurement data, the metal wire diameter is deduced using　the traditional calculation method and the programming solution method respectively. By comparing the results from the two methods, the programming solution method has a room for further improvement in measuring accuracy. This work not only improves the calculation accuracy of metal wire diameter, but also provides a guidance on the measurement of metal wire diameter.