Abstract: A simulation was conducted on the collision and coagulation of particles flowing in the flue duct before electrostatic precipitator of a power plant under full load conditions using Eulerian-Lagrangian method and O'Rouekr collision model. Results show that, the higher the flue gas velocity is at the coagulator inlet, the larger the turbulent intensity and vorticity will be, which is favorable for the reduction of small particles, but in a limited degree; whereas, the higher the flue gas velocity is at the coagulator inlet, the shorter the residence time of particles will be in the coagulation zone, and the smaller the particles will be formed by collision and coagulation, which is unfavorable for the formation of large particles. Within the common range of flue gas velocity in a power plant, the increase of flue gas velocity at the coagulator inlet has a negative impact stronger than positive impact on the turbulence coagulation.