Studying the effect of co-combustion of multiple fuels on the soot formation has become a hot spot in the investigation of soot particles. In this paper, the influence of methane blending on soot formation in ethylene flame combustion is studied experimentally and numerically. The visible spectrum of flame image processing technology was used to in situ measurement of laminar flame temperature and carbon smoke volume points in the experiment. The effects of different methane blending ratios on particle nucleation, coalescence, surface growth and oxidation process of soot were analyzed based on the piecewise particle dynamics soot model of polycyclic aromatic hydrocarbons (PAHs) by using CoFlame Code. Results indicate that the synergistic effect promoted the increasing rate of nucleation and addition reaction of hydrogen extraction at low methane blending ratio, and the increase of the total mass of soot was mainly due to PAH condensation rate. The total amount of soot generation gradually decreases with increasing blending ratio. The overall trend of condensation, surface growth rate and soot nucleation in the flame decreases with increasing blending ratio. And the nucleation rate gradually shifts from a single peak to a double peak and increases slightly at the initial stage of the flame combustion reaction. It is worth mentioning that the change of three PAH precursor (BAPYRS, BAPYR and BGHIF) and the temperature explains the change of nucleation rate from unimodal to bimodal.