The soot formation characteristics of mixed diffusion flame of methane doping into ethylene/hydrogen were explored with Laser Confocal Raman spectrum (Raman) and Fourier Transform Infrared Reflection (FTIR), the soot order degree and the functional group’s distribution characteristic in the mixed flame with different proportions of methane doping were investigated. The influence of methane doping on soot formation of ethylene/hydrogen (30% hydrogen) laminar diffusion flame was studied. Results show that the order of soot in the range of flame height less than 4 cm was significantly reduced when the methane mixing ratio was 3% and 7% respectively, indicating that there is an obvious synergistic effect in this region; when the methane doping ratio increased by more than 10%, the synergistic effect disappeared and the order degree of soot increased. The influence of methane on the composition of soot functional groups was obvious. After being mixed with methane, the relative content of aliphatic functional groups increased overall. With the gradual increase of the doping ratio of methane, the relative content of CH₂ reached the peak value. When the proportion of methane continued to increase, the relative content of CH₂ decreased. The content of aromatic functional groups in soot decreased significantly with the increase of flame height. When mixed with 3% and 7% methane, the content of aromatic functional groups increased significantly at the flame height of 2 and 3 cm, respectively. When mixed with over 10% methane, the content of aromatic functional groups decreased, which showed that a small amount of methane doping provided a new way for CH₃ and C₃H₃ generation. While CH₃ and C₃H₃ increased, C₂H₄ and C₂H₂ decreased insignificantly, which promoted the formation of PAHs. Continuous increase of methane reduced the formation of C₂H₂ and the formation of PAHs accordingly; as the relative content of aromatic decreased, the formation of soot was reduced. This study reveals the interaction between methane and soot formation in ethylene / hydrogen laminar diffusion flame. There is a synergistic effect to promote soot growth at low a methane doping ratio, but it disappears at a high methane doping ratio.