Abstract: The CoFlame program was used to simulate the effect of hydrogen addition on soot formation in ethylene/air diffusion flame. A gas phase reaction mechanism with the soot precursors and its complex thermal and transport properties were utilized to couple soot model, and the distribution of flame temperature, soot volume fraction, mole fraction of important species of soot and important parameters describing soot growth process was obtained. The effects of chemical effect, dilution effect and thermal effect of hydrogen on soot formation were analyzed. The results show that the flame temperature of ethylene with hydrogen addition has little change, and the thermal effect of hydrogen (related to temperature change) is not the major factor affecting soot formation. Adding hydrogen can effectively slow down nucleation rate, surface growth rate, condensation rate and number density due to reducing the mole fraction of soot precursors acetylene, benzene and pyrene, which finally inhibits the soot formation.