Abstract: Carbon dioxide pumped thermal energy storage systems do not require the storage of carbon dioxide, eliminating the dependency on large gas storage facilities. Different types of carbon dioxide pumped thermal energy storage systems exhibit various performance. In view of this, a comparative study of the thermodynamic performance of carbon dioxide pumped thermal energy storage systems based on the Brayton cycle and the Rankine cycle was conducted. The parameters of both systems were optimized according to the results of the sensitivity analysis. Results show that under basic operating conditions, the Rankine cycle-based carbon dioxide pumped thermal energy storage system has a higher cycle efficiency of 51.86%, and reducing the inlet pressure and compression ratio of the high-temperature compressor can enhance its cycle efficiency. The Brayton cycle-based carbon dioxide pumped thermal energy storage system has a higher energy storage density of 7.05 kW·h/m³, and increasing the inlet temperature of the high-temperature compressor can improve its cycle efficiency. After parameter optimization, the cycle efficiency of the Brayton cycle-based carbon dioxide pumped thermal energy storage system increases by 13.18%, while the cycle efficiency of the Rankine cycle-based system increases by 11.09%.