In order to study the effect of carbon nanotubes (CNTs) on the flame retardancy of carbon fiber (CF)/epoxy resin (EP) composites, CF/EP and CNTs/CF/EP composites were prepared by solution blending. The flame retardancy and thermal stability were studied by cone calorimetry and thermogravimetric analysis. It was found that CNTs and CF had a certain synergistic effect on improving flame retardancy and thermal stability of EP. The peak heat release rate of F7N7, which represents the EP composites with 0.7 wt % CF and 0.7 wt % CNTs, was minimal. The total smoke production of F5N5 which represents the EP composites with 0.5 wt % CF and 0.5 wt % CNTs was the smallest, which was decreased by 43.04% more than the EP. The initial decomposition temperature of F7N7 was about 14 °C higher than that of F7, and the mass loss at T_max was greatly reduced. The apparent activation energy of F7N7 is 2.7 kJ·mol⁻¹ more than EP. Finally, the tensile and flexural strength of the composites were also improved, so it could be applied to a high-performance matrix of CF/EP composites, which are usually used as the advanced composites in the aerospace field.