As an environment-friendly refrigerant,CO2 has been paid a lot of attention again in the area of air-conditioning nowadays.In this paper,the CO2 trans-critical refrigeration cycle was analyzed thermodynamically. It was found:there is an optimal discharge pressure at which COP reaches maximum;It is useful to enhance COP for cycles by increasing the evaporation temperature or decreasing the cooling pressure,though the exergy efficiency is reduced;In the practical working system,the outlet temperature of the gas cooler should be de-creased as much as possible,or increase the evaporator temperature.%环保工质CO2作为制冷剂用于空调领域再次受到广泛关注。文中对CO2跨临界循环进行了热力学理论分析，分析结果表明：循环系统存在最优高压压力，使得其COP 达到最大值；蒸发温度的升高或者冷却压力的降低都能提高COP ，但都会降低效率；实际运行系统中，应该尽可能提高蒸发温度或者降低气体冷却器的出口温度。
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