The influence of flue gas composition on the specific resistance of coal-fired fly ash is studied in this paper. We conclude that the negative electrons of NO2 and SO2 gases are strong. The probabilities of electron desorption on SO2 and NO2 negative ions are lower than that in air atmosphere at high temperature. Therefore, the introduction of SO2 causes NO2 to reduce the specific resistance value of coal-fired fly ash. When the pores on the surface of fly ash particles are filled with SO2, no change will occur in NO2, average pore size, pore volume, and specific surface area of fly ash particles, thereby resulting in fly ash that remains unchanged from the resistance value. When humidity increases, the surface conduction effect is greatly enhanced, and the specific resistance value is lowered considerably. Therefore, the specific resistance of dust can be reduced by humidification.