Running fractures in long-distance CO2 pipelines are considered catastrophic pipeline failures and can result in the rapid tearing of the pipeline for several hundred metres and the release of massive amounts of inventory within a short time. The prediction of inventory pressure response and phase transition in the event of accidental pipeline rupture is of paramount importance to determining fracture behaviour in a CO2 pipeline. In order to simulate an actual CO2 pipeline, a large-scale experimental pipeline with a total length of 258 m and the inner diameter of 233 mm is developed to study the fluid dynamic behaviour of CO2 pipeline blowdown. High frequency transducers were used to measure the evolution of fluid pressure after rupture. Thermocouples on the top and bottom of pipeline were installed to monitor the temperature distributions inside the pipeline. The pressure responses and phase transitions of supercritical CO2 were studied following pipeline rupture with three orifice diameters (15 mm, 50 mm and Full Bore Rupture). The waveform characteristics of the pressure response and the pressure change rate were studied in supercritical leakage with different orifice diameters, which could be applied to ascertain the leakage location and the leakage diameter size in the real-time monitoring of CO2 pipeline. (C) 2016 Elsevier Ltd. All rights reserved.