As a natural refrigerant, carbon dioxide (CO2, R744) is proposed to be used in chillers and heat pumps. Generally, CO2 in chillers and heat pumps is a trans-critical cycle. CO2 is super-critical in the evaporator of trans-critical cycle, which is called gas cooler. The CO2-water gas cooler of helix tube was designed and studied in the paper. An experimental research was conducted in range of CO2 temperature (373.15-393.15 K) and CO2 pressure (8-10 MPa). The numerical simulation was conducted to investigate the temperature distribution and flow velocity distribution in the gas cooler. Based on the experimental and numerical study, the results showed that the helix tube gas cooler had a good field coordination between the temperature gradient and flow velocity. The experimental results show that the Fc (field coordination number) of fluids in the helix tube gas cooler was less than other gas coolers, especially in the water side. The numerical results show that the distribution of cooling water temperature in the helix tube gas cooler was more complex and inhomogeneous than the straight tube gas cooler. The heat transfer performance of the helix tube gas cooler was investigated experimentally. An increase of CO2 bulk temperature deteriorated the heat transfer performance due to a decrease of thermal properties (e.g., specific heat and thermal conductivity). The exergy loss rate was used to evaluate the irreversibility of gas cooler. The results show that the order of irreversibility is straight tube, single helix tube, followed by double helix tube.