Compared with traditional oil and gas resources, geothermal energy processes the advantages of zero emission, low investment and high productivity, etc. in this paper, a model is developed for simulating CO2 circulating in a geothermal horizontal well. The energy and momentum balance equations of CO2 flow in long tube and annuli are solved numerically. The relationships between geothermal productivity and injection parameters are revealed. It is concluded that: (a) The standard of choosing parameters for measuring the rate of geothermal production is whether or not the parameter can reflect the heat transfer rate or fluid temperature rise. (b) Taking multiple factors (temperature decrease, enthalpy decrease, small increase in heat production rate and increase in cost) into consideration, a small mass injection rate is recommended to obtain a better economic performance. (c) Taking multiple factors (temperature increase, decrease in heat production rate and increase in cost) into consideration, a small injection pressure is recommended to obtain a better economic performance. (C) 2018 Elsevier Ltd. All rights reserved.