Using existing natural gas pipelines for hydrogen transportation has become a hot topic in the field of hydrogen energy and natural gas delivery. Currently, researches on the leakage and explosion of hydrogen-blended natural gas pipelines have primarily focused on enclosed spaces, with little exploration for long-distance pipelines. To investigate the laws governing leakage and explosion in hydrogen-blended natural gas pipelines, this study employs a mathematical model and PHAST calculations to a real-world example of a long-distance hydrogen-blended natural gas pipeline. The patterns of leakage and diffusion in the hydrogen-blended natural gas pipeline are examined under various operating pressures, hydrogen blending ratios, leak hole sizes, weather conditions, pipeline operating temperature and leak orientation. The results indicate that: ① The leakage diffusion range is positively correlated with the operating pressure and leak hole size, while negatively related with the pipeline operating temperature. The diffusion area is also affected by the hydrogen blending ratio and leak orientation, in a positive or negative way depending on the values of the two factors. ② The thermal radiation range is positively related with the operating pressure while negatively correlates with the hydrogen blending ratio and pipeline operating temperature. The findings of this research theoretically clarifies the leakage characteristics of hydrogen-blended natural gas pipelines, expected to provide important guidance for safe transportation and accident handling of hydrogen-blended natural gas.