The energy storage device is indispensable in future power system due to the high share of renewable energies. Compressed gas energy storage (CGES) technology, including the compressed air energy storage (CAES), has been widely recognized as one of the most promising solutions in energy storage domain. Among multiple CAES forms, the isothermal CAES is a potential type in which the ideal round trip efficiency (RTE) can be 100%. On the other hand, carbon dioxide (CO₂) as an excellent working medium in CGES system owing to its favorable properties. However, the existing isothermal type CGES systems just employ the air as working medium, the potential of CO₂ in such system is not evaluated yet. Therefore, an 800 kW pumped hydro assisted near-isothermal compressed carbon dioxide energy storage system with gas/liquid phase change process is proposed. In detail, the hydraulic machineries, the flexible rubber diaphragm and the helical coils are employed to realize the near-isothermal process and high RTE. The transient models are built and system performance is thus investigated. The results show that the system possesses 68.36% in RTE and 1.0914 kWh/m³ in energy density (ED) under the design condition. Meanwhile, the water temperature in helical coil, the initial and maximum pressures of vessel and the vessel volume are sensitive for system behavior.