Abstract:  Objective There is a phenomenon of different inlet temperatures between hydrogen and natural gas in existing hydrogen doped natural gas pipelines, and conventional turbulence simulations are often used. The aim is to reveal the mixing law of hydrogen doped natural gas pipelines through large eddy simulation, analyze the heat and mass transfer phenomena of hydrogen doped natural gas, and provide theoretical basis for optimizing pipeline design, improving safety and mixing efficiency. Methods A T-shaped mixing pipeline model was constructed based on ANSYS Fluent, and the turbulent characteristics during the flow process were more accurately captured using Large Eddy Simulation (LES). The k-ω turbulence model was used for steady-state initialization and transient calculations, and the mixing characteristics under different operating conditions were systematically analyzed. Study the effects of injection methods (horizontal, top side vertical, bottom side vertical), pipeline diameter ratio, and hydrogen blending ratio on mass transfer efficiency; Study the influence of injection angle and mixed flow state (wall jet, deflected jet, impact jet) on heat transfer efficiency. Quantify the mixing uniformity through coefficient of variation (CoV), and evaluate the thermal mixing effect and fatigue risk by dimensionless temperature (T^*) and thermal stress model. Results In terms of mass transfer, vertical bottom injection benefits from buoyancy effect and can mix evenly in the shortest distance; The optimal mixing efficiency is achieved when the diameter ratio of the main pipe to the branch pipe is 4:1-5:1; A hydrogen doping ratio exceeding 15% is prone to flow stratification. In terms of heat transfer, vertical injection from the bottom can achieve efficient thermal mixing, but there are strong thermal fluctuations near the pipe wall, and attention should be paid to protection in the near wall area; Although the impact jet promotes temperature mixing, it increases the risk of hydrogen corrosion on the wall. The deflected jet strikes a balance between safety and mixing efficiency, while the wall jet has the worst thermal mixing effect. Conclusion The engineering design recommends the bottom vertical injection method, with a pipe diameter ratio of 4:1~5:1, a hydrogen blending ratio of ≤15%, a reserved mixing section length of ≥15D (D is the main pipe diameter), and preferably a deflected jet to reduce thermal stress. Attention should be paid to pipe wall protection downstream of the mixing section from 0.5D to 6D. This study provides key parameter guidance for the safe design and efficient operation of hydrogen doped natural gas pipelines, which is of great engineering significance for promoting the large-scale transportation of hydrogen gas.