The marine environment has abundant natural cold energy; using the CO2 closed cycle combined for development is considered one of the best options. However, seawater temperature fluctuates greatly with depth and can reach 4–6 °C at a depth of 1000 m; the optimal configuration and parameter matching of the CO2 closed cycle suitable for this environment are still a mystery. Therefore, this chapter established the thermodynamic model to investigate the performance of CO2-based mixed fluid cycle under variable low-temperature seawater conditions. The results indicated that the transcritical gas-phase CO2-based mixed fluid Brayton cycle (TGBC) and the supercritical CO2-based mixed fluid Brayton cycle (SBC) are more suitable for low-power level systems on seawater surface, while the transcritical liquid-phase CO2-based mixed fluid Brayton cycle (TLBC) and the transcritical CO2-based mixed fluid Rankine cycle (TRC) are more suitable for deep-sea high-power level systems. The low critical parameter mixed working fluid cycle can further expand the cycle temperature range and pressure ratio to improve the cycle thermodynamic performance. Considering the cycle performance, the thermal efficiency of the CO2/Xe (0.5/0.5) transcritical Rankine cycle, the CO2/SF6 (0.9/0.1) transcritical liquid Brayton cycle and the CO2/SF6 (0.5/0.5) transcritical Rankine cycle can be increased by 3.79% compared with the S-CO2 Brayton cycle. This work provides insights into the application of CO2-based mixed fluid cycle in marine environments and subsequent research priorities.