A laboratory scale experimental pipeline system is developed to investigate the leakage behavior of high pressure CO 2 . The 23‐m‐long experimental pipeline could provide an inner pressure varying from 0.1 to 10 MPa and a preset temperature within the range of 0 ∼ 50 °C to ensure a stable initial phase state for the leakage test. Experiments on CO 2 leakage with three initial inner pressures and four different nozzle orifice sizes were conducted using this system. The individual and combined effects of inner pressure and leakage orifice size on the main characteristic flow parameters of the pipeline, the structure of the leakage jet, and the temperature decrease of the plume were investigated. Based on the experimental results, limitations of the previous theoretical formula on mass loss rate were discussed. The maximum temperature decrease along the centerline of the far‐field plume by Joule‐Thomson effect was also studied. The new experimental test facility proves to be valuable in analyzing the hazardous leakage process of CO 2 from pipelines.