Geothermal energy is abundant and renewable. It is of great interest to extract geothermal energy efficiently and sustainably. In this paper, a composite coiled tubing (CCT) is put forward and a coaxial borehole heat exchanger (CBHE) technology is utilized to extract heat from the medium-deep geothermal reservoir using phase-change CO2 as the working fluid. CCT has all the advantages of coiled tubing, and thermal insulation could be realized more easily. Considering the transient heat transfer of the formation, a mathematical model of the CBHE system is established and validated by two field tests. Then, both the phase change process of CO2 in the CCT and the long-term heat extraction performance are predicted. Effects of key parameters on the heat output are discussed. It is shown that extracting medium-deep geothermal energy by the phase-change CO2 is realized, and the latent heat of vaporization accounts for a high proportion of the heat carried by production CO2. The larger the mass flow rate is, the more heat could be carried out. But the larger flow rate would lead to the lower temperature of the production CO2 and the more frictional loss. The results provide a reference for exploiting the medium-deep geothermal energy sustainably.