Abstract: Blending hydrogen into natural gas is the primary method to solve the mismatch between the origin and use of hydrogen and realize the large-scale and long-distance transportation for hydrogen. Hydrogen can lead to hydrogen embrittlement in natural gas pipelines and cause safety accidents. Therefore, it is necessary to investigate the regularity analysis of hydrogen composition, velocity, and accumulation in in-service natural gas pipelines. A T-type and a variable diameter type blending pipeline model have been constructed and researched for blending hydrogen in natural gas. The T-type results show that the pipe length of obvious layering and width are 35 times and 1/3 of pipe diameter, respectively. The results of variable diameter have been illustrated that the closer the blending center, the narrower of width and the lower of height with variable diameter can lead to hydrogen enrichment easily. Hydrogen embrittlement has been facile with a maximum hydrogen mole fraction of 50%~60% in blending nature gas pipelines. The results can provide a reference for the blending effect of natural gas with hydrogen and the selection of pipeline diameter.