In this paper,first,a mathematical model based on the particle population balance theory,which takes into consideration the nucleation,coagulation,surface growth and oxidation of particles,is established to describe the dynamic evolution of soot particles in one-dimension laminar premixed flames.Next,the model is solved by means of Monte Carlo stochastic method.Then,based on a detailed chemical kinetic mechanism,a computation platform of particle size distribution is established.With this platform,the effects of CO₂ and H₂O addition on the soot formation in laminar acetylene /air premixed flames are analyzed,and the particle size distribution affected by the CO₂ /H₂O dosage ( 0%,20% and 40%) is obtained8.The results show that the addition of CO₂ and H₂O slows down the soot formation when the equivalence ratio remains unchanged at 2. 5,especially at a H₂O addition of 40%; and that fewer particles in large size may form due to the addition of CO₂ and H₂O.