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Investigating the transient behaviour of liquid CO2 decompression is of great importance to ensure the safety of pipeline transportation in carbon capture and storage (CCS) technology. A computational fluid dynamics (CFD) decompression model based on the non-equilibrium phase transition and Span–Wagner equation of state (EoS) was developed to study the effects of actual flowing state within the pipeline on the transient behaviour of liquid CO2 decompression. Then, the CFD model was verified by comparing the simulated results to test data of a large-scale “shock tube” with an inner diameter of 146.36 mm. The results showed that the evaporation coefficient had a significant impact on the transition behaviour of CO2 decompression, while the condensation coefficient made no difference. When the evaporation coefficient was 15 s−1, the CFD-predicted results were in good agreement with the test results. Moreover, the effects of flow velocity on transient behaviour of liquid CO2 decompression were further investigated. It was found that the flow velocity affected the temperature drop of liquid CO2 during decompression, thereby affecting the phase transition of liquid CO2. In addition, the initial flow velocity also showed a significant influence on the transient behaviour of CO2 outside the pipe.