The pipeline leakage process of supercritical CO2 involves complex phenomena because of the high inner pressure and the multiphase choked flow near the leakage point. In this work, an experimental facility was developed to investigate the thermodynamic and fluid dynamic behaviour in pressurized CO2 leakage process. Characteristics of the flow and heat transfer in the leakage processes were studied by investigating the variation of the mass flow rate and the development of the thermal boundary layer in the pipeline. Inner pressure, mass outflow rate at the leakage nozzle and Nusselt number in the pipeline were studied quantitatively based on the laboratory pipeline leakage system. Typical rapid expansion behaviour of supercritical fluid including sonic-choked flow phenomena and phase-transition process was observed during the leakage process. The data of the mass flow rate and Nusselt number could be used for leakage detection and validating numerical simulations of supercritical-gas multiphase flows in the pipeline leakage process.