Abstract: The nitrogen purging & packing of gas pipeline commissioning is mainly determined depending on the experience, so there is great blindness. With FLUENT software, the convective diffusion course of nitrogen in air at different working conditions is simulated during the nitrogen purging & packing course of different gas pipelines. The influencing factors of gas-mixing length are analyzed.Resultsshow that flow pattern, back pressure, flow rate, pipeline diameter and pipeline length are five influencing factors in the gas-mixing length. The gas-mixing length grows with the increase of back pressure, flow rate, pipeline diameter and pipeline length. The corresponding gas-mixing length and time for the nitrogen are longer when it is at laminar flow. The simulation results are consistent with the experimental results. The research results are instructive for optimizing the commissioning of gas pipelines.