CO2 pipeline is the key link between CO2 emission site and storage or injection site in CCUS technology. The main purpose of CO2 pipeline commissioning replacement is to clean and dry the pipeline and prepare for the subsequent commissioning of the pipeline by using the characteristics of inert and highly absorbent N2. In this paper, the process of CO2 pipeline commissioning replacement is studied by SPS software under different conditions of transport fluid mixture composition and storage pressure. The results show that the CO2 density increases abruptly when the pressure rises near the CO2 critical point, and the gas volume required per unit of pressure increasing reduces, and the CO2 consumption is smaller than the natural gas consumption at the end of the ramp-up. The change of fluid mixture composition in the pipe has little effect on nitrogen replacement, and CO2 is more likely to undergo phase change in the pipeline, so it is not suitable to put into operation directly after the replacement is completed.