The emission of fine particles, especially those with an aerodynamic diameter of less than 2.5 mu m named PM2.5, from coal combustion is an important source of atmospheric PM2.5. To improve the removal of PM2.5 by an electrostatic precipitator (ESP), an agglomerant solution prepared by dissolving agglomerant in process water or desulfurization wastewater was sprayed at the inlet of the ESP. The number concentration and diameter distribution of the particles were investigated before and after the agglomeration solution addition based on a coal-fired thermal system, and the effects of the operating parameters, such as the species and concentration of the agglomeration solution, flue gas temperature, pH value of the agglomeration solution, and diameter of the spray droplets, on the fine particle removal efficiency were analyzed. The results show that the average diameter of the particles could grow more than 4 times as a result of the effects of wetting, the liquid bridge force, and adsorption bridging, and the PM2.5 concentration at the ESP outlet could decrease by 40% under typical flue gas conditions. The removal efficiency of the ESP on fine particles can be increased with a high concentration and low pH value in the agglomeration solution. The concentration of fine particles changes slightly at the outlet of the ESP when only spraying desulfurization wastewater, but it decreases considerably when the desulfurization wastewater contains agglomerants.