Commercial refrigeration systems in supermarkets are intensive users of energy and CO2 is recognized as the most promising refrigerant such systems. The CO2 parallel compression system has been considered in medium- and low-temperature supermarket refrigeration systems, with auxiliary compressors to enhance system performance. This study is intended to investigate the effects of adding a subcooler in a CO2 trans-critical supermarket refrigeration system with parallel compression (SPR system). Mathematical models based on mass and energy conservation are built. Optimization is performed for the gas cooler pressure and the receiver pressure for maximizing the coefficient of performance (COP). The performance of the parallel compression system with a subcooler is analyzed and compared to a baseline parallel compression system in different cities of China. A 6.8% average promotion at least in seasonal energy efficiency ratio (SEER) is predicted, and the energy consumption of the whole system decreases 7.1% totally in Haikou. Gas cooler, taking up 63.3% of the total exergy destruction rate, is the key component to increase efficiency through exergy analysis.