The mixed transmission of natural gas with injected hydrogen is an important means of transporting the green hydrogen energy at a lower cost. The mixing process of injecting hydrogen with different concentration in natural gas is numerically simulated in this research using the computational fluid dynamics method. The effects of the injection position and size of injection tube and the injecting flux of hydrogen on the required length to achieve uniform mixing are examined. Compared with injection from the upper or lower part of the pipeline, injection from the pipe axis resulted in the shortest distance to achieve uniform mixing; as this is dependent on the turbulent intensity downstream the injection point. When injection from the axis of the pipe, the turbulent intensity is enhanced with the increase of the diameter of injection tube or the decrease of injecting flux, resulting in the shortened required length for uniform mixing. Therefore, when operating condition permits, it is recommended to arrange the injection tube at the pipe axis and to enlarge its size appropriately so as to achieve faster and uniform mixing.