In order to optimize the energy structure and reveal the evolution law of gas plume diffusion caused by leakage in hydrogen-blended submarine pipelines, a numerical model for leakage and diffusion of hydrogen-blended submarine pipelines was established using FLUENT software based on multiphase flow dynamics theory and computational fluid dynamics theory. Multi-condition simulations were conducted, revealing that the diffusion process of leaked gas plumes undergoes four characteristic stages. Special focus was placed on investigating the influence mechanisms of hydrogen blending ratio, leakage orifice diameter, ocean current velocity, and seawater depth on the diffusion patterns and migration paths of hydrogen-blended natural gas leakage plumes. The results demonstrate that hydrogen blending ratio, leakage orifice diameter, ocean current velocity, and seawater depth all exert significant effects on the diffusion process of leaked gas plumes from hydrogen-blended submarine pipelines. A higher hydrogen blending ratio leads to a larger diffusion range of the hydrogen-blended natural gas plume and a shorter time to reach the sea surface. When the leakage orifice diameter is small, the gas plume tends to exhibit unsteady fluctuations, and the longitudinal diffusion velocity slows down as it approaches the sea surface. Conversely, a larger leakage orifice diameter results in a relatively stable upward floating process of the gas plume and a shorter time to reach the sea surface. A higher ocean current velocity causes a greater diffusion deflection distance of the gas plume along the current direction, forming a curved migration path that varies with the velocity gradient. Additionally, The greater the distance from the leakage orifice to the sea surface, the longer the time required for the gas plume to rise to the sea surface.