The numerical simulation of straight tube air cooler is carried out by using FLUENT software. The operation pressure, the mass flow rate of carbon dioxide, the mass flow rate of cooling water and the inlet temperature are changed for numerical calculation, the temperature distribution of carbon dioxide and cooling water along the length of the pipeline is obtained. According to the second law of thermodynamics, the distribution of entropy generation along the pipe length is obtained by calculating the thermal process in straight pipe casing. The results show that, with the increase of the pressure, the entropy production along the length of the pipe increases gradually; with the increase of the mass flow rate of carbon dioxide, the entropy production decreases; with the increase of the mass flow rate of cooling water, the entropy production does not increase significantly; with the increase of the inlet temperature of the cooling water, the entropy production decreases.