In order to improve the accuracy of fly ash permeability calculation，firstly，the diameter and number of pores and particles in fly ash specimens are obtained by using microdigital imaging technology and professional image processing technology. Then，the frequency histogram of pore and particle is plotted，and the pore exponential distribution function and particle Rayleigh distribution function are fitted by the least square method. Secondly，the pores and particles are divided into three grades，and their characteristic particle sizes are calculated and arranged in order from small to large. Finally，based on the ball-touch process without dropping back，Matlab programming was used to simulate the random arrangement of pores and particles in a square with a diameter of 5 times the maximum particle equivalent circle，and the connectivity of pores was calculated. On this basis，the formula of fly ash permeability considering the porosity connectivity is derived，which is compared and analyzed with the results of other literatures. The good agreement proves the feasibility of the method in this paper.