Abstract: In the view of phase behavior control and taking the CO₂ emissions of onshore terminal of Dongfang 1-1 Gasfield in Hainan Island as an example, an optimization research into transmission pressure and pipe diameter of longdistance CO₂ transmission pipeline is carried out, and the sensitivity analysis for pipeline inlet pressure, inlet temperature and ambient temperature is performed. The results show that the transmission pressure has an important influence on CO₂ transmission phase behavior, which will largely determine the diameter and wall thickness of pipeline to a great extent, and thus affect pipeline economics. The pipeline transmission pressure should be maintained above the CO₂ critical pressure and CO₂ should always be in high pressure dense phase state (supercritical or liquid state). It is recommended that the pipeline transmission pressure should be 9 MPa or 16 MPa and X65 steel pipe material be used for long-distance transportation plan of associated CO₂ of Dongfang Gasfield, and thermal insulation is not required.