Abstract: In CCUS industry chain, supercritical CO₂ pipeline transportation gains growing popularity due to its high efficiency and economic performance. Any leakage or pipe rupture of high-pressure large-flow CO₂ pipelines leads to heavy economic losses and environmental risks. Line block valves act as core safety equipment. Their automatic rapid closure performance directly determines the control effectiveness of pipeline accidents. Systematic research on the set values of shutoff pressure drop rate for line block valves of supercritical CO₂ pipelines is still lacking. This paper establishes a numerical model describing CO₂ pipeline leakage and pressure relief. The reliability of this model is verified through two independent sources: published experimental data and high-pressure visual tests. With the parameters of a million-ton-scale supercritical CO₂ pipeline demonstration project as the basis, simulation calculations are carried out for different pipeline segments and three leakage aperture sizes (20, 40, 237.1 mm). The simulations capture the pressure variations and pressure drop rates at each block valve, enabling a quantitative analysis of pressure wave variation and transmission characteristics. This study proposes a methodology to determine the pressure drop rate threshold for block valve closure. The derived reference thresholds for the three leakage apertures are 0.403 7 MPa/min for 20 mm, 1.515 2 MPa/min for 40 mm, and 4.499 7 MPa/min for 237.1 mm. Accounting for all scenarios, the 20 mm small-aperture leakage is established as the primary design boundary. Therefore, the final recommended reference threshold of the pressure drop rate for block valve closure is set at 0.40 MPa/min. The research findings provide fundamental data and references for setting the pressure drop rate for line block valve closure on long-distance CO₂ transmission pipelines.