Abstract: Geometric morphology and soot characteristics are fundamental properties of hydrocarbon fuel diffusion flames. Investigating laminar diffusion flame behavior under microgravity conditions provides a crucial approach for elucidating the physical and chemical mechanisms of diffusion combustion and for establishing turbulent diffusion combustion models. On-orbit microgravity experiments were conducted on coaxial co-flow methane laminar jet diffusion flames using the combustion science experiment cabinet aboard the Chinese Space Station. The study focused on analyzing the influence of co-flowing gases on morphological characteristics and soot properties of microgravity flames. Experiments were conducted under ambient temperature and pressure conditions. The co-flowing gases comprised nitrogen-oxygen mixtures with varying oxygen concentrations and air diluted with different ratios of N₂ and Ar. The ratio of entrainment velocity to jet velocity was maintained below 0.5, with methane flow rates generating both far-field and near-field jet flames. Results indicate that a simplified model based on jet flow field similarity theory can effectively predict the shape of microgravity flames in the far-field region of the jet. Co-flow composition affects the flame shape by altering the combustion stoichiometry. Near-field flame length is independent of the co-flowing velocity but inversely proportional to the stoichiometric mixture fraction Z_st, while the maximum flame diameter is proportional to the square root of the inverse of Z_st. When diluting the co-flowing air with an inert gas, the region dominated by soot formation in the jet diffusion flame decreases, while the region dominated by soot oxidation increases. As dilution increases, the soot content within the flame decreases. The effects of dilution and thermal effects on soot formation are characterized by the volume fraction of the inert gas and the flame temperature, respectively. Diffusion flames in microgravity provide an ideal research subject for understanding fundamental combustion processes and mechanisms. The findings of this study offer foundational data for elucidating basic combustion phenomena and advancing combustion theory, holding significant importance.