Tubular string temperature and pressure are important parameters in string mechanics analysis. Thus, accurately calculating temperature and pressure in the injection process is fundamental for analyzing the tubular string mechanics of CO₂ injection wells. Based on the S-W equation of state and Vesovic models, we modeled the physical properties of CO₂. Then, based on the physical properties of CO₂, combined with the theory of heat transfer and three conservation laws, a temperature–pressure coupling model of a tubular string was established. Lastly, the temperature and pressure field distributions of the G66X1 well were determined using the alternating iteration method. According to a comparison of the established model and the actual data, the maximum error in predicting temperature was 4.1% and the maximum error in predicting pressure was 2.3%; thus, the model exhibits a high level of accuracy. In the final section, the model was used to study the influence of injection temperature on the tubular string temperature and pressure field distribution. Next, we studied the influence of the injection time, displacement, and pressure on the bottom hole temperature. This study provides a reference for predicting the wellbore temperature and pressure in CO₂ injection wells.