Supercritical CO2 in vertical tubes has been widely studied, but limited work exists for mixed CO2 fluids. In this study, the heat transfer characteristics of supercritical CO2/R134a mixtures in a vertical circular tube with an inner diameter of 8 mm were experimentally investigated. The experimental conditions were as follows: the operating pressure of 8.0–9.4 MPa, the mass flow rate of 750–1500 kg·m−2·s−1, and the heat flux of 20–260 kW·m−2. The results indicate that the addition of R134a to pure CO2 significantly alters its thermodynamic properties, leading to an increase in both wall and bulk temperatures while weakening heat transfer performance. The effects of mass flux, heat flux, and pressure on heat transfer are complex: increasing heat flux raises the temperature but decreases the heat transfer coefficient, while higher mass flux improves the convective heat transfer coefficient and causes only a slight increase in the inner-wall temperature. The effect of pressure is minimal, especially at pressures far from the critical value. The heat transfer behavior of CO2/R134a mixtures is found to be consistent with that of pure CO2, and the Mokry correlation is recommended for predicting supercritical heat transfer in CO2/R134a mixtures. This study provides valuable database for optimizing the design and performance of CO2-based power cycles, offering potential improvements in energy efficiency and system adaptability.