In order to explore the calculation method of porosity and pore multi-fractal characteristics of fly ash，the concept of similar dimension is introduced to derive the basic formula of fractal dimension. By processing the sample mold to control the degree of compaction，a fly ash sample with a certain moisture content is configured. Secondly，the diameter and number of pores and particles in the fly ash sample are obtained by using digital microscopic imaging technology，and then the histogram of pores and particles can be obtained. Then 4 different depth sections are selected，whose upper and lower ends are used as the observation surface，and photos at different magnifications are taken with the help of a stereo microscope to analyze the distribution of the pore area ratio in the three-dimensional space；finally，professional image processing is used. The technique gives the porosity of the sample and compares it with the measured results. The research results show that the pore area ratio of the fly ash has a decreasing trend along the depth direction，the variation of which would be about 30%，and its pore distribution has obvious multifractal characteristics.