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The removal of nonconventional pollutants in coal-fired power plants, such as SO3, has been receiving more and more attention. However, due to its unique nature, it is difficult to remove SO3 effectively with the widely used wet flue gas desulfurization systems. Nowadays, dry-sorbent injection technology has become a promising method for SO3 emission control in coal-fired power plants. The removal characteristics of SO3 from flue gas with modified fly ash adsorbents were investigated in a fixed-bed reactor. Factors affecting the adsorption efficiency of SO3 were studied, including modification method, modified fly ash adsorbent particle size, reaction temperature, and flue gas component. Combined with adsorbent characterization analysis, the adsorption kinetics of SO3 by modified fly ash adsorbents were carried out with four different adsorption kinetics models. The results show that the SO3 adsorption efficiency of the fly ash samples increases after modification; the best SO3 removal performance of fly ash was achieved when 1.5 mol/L NaOH solution was used, with the highest SO3 adsorption efficiency of up to 98.3%. The modified fly ash adsorbent particle size, water vapor content, and the addition of NO have little effect on the adsorption efficiency of SO3. As the reaction temperature increases from 250 to 450 °C, the SO3 adsorption efficiency first increases and then decreases, with an optimal reaction temperature of 350 °C. The addition of SO2 would compete with SO3 for adsorption and inhibit the uptake of SO3 by the adsorbent. Adsorption kinetics data show that external mass transfer and chemical adsorption are the main critical mechanisms affecting the adsorption efficiency of the modified fly ash adsorbent in the SO3 removal process compared to internal diffusion.