Copolymer thermoplastic polyurethane (C-TPU) was extruded with intumescing flame-retardant formulations based on ammonium polyphosphate and macromolecular nitrogen phosphorus. Carbon nanofiber and carbon nanotube were used as the additional carbon source. The synergism effect of each additive and their intumescing combinations on C-TPU composites degradation, smoke suppression, flammability, and melt rheology was systematically investigated by thermogravimetric (TG), smoke density test and cone calorimeter test (CCT), etc. The TG results showed that carbon particles combined with IFR showed a notable improvement in thermostability at high temperature, and this intumescing flame-retardant system effectively catalyzed the decomposition of macromolecule volatiles that is the major source of smoke particles. This intumescing flame-retardant system also promoted the generation of compact and continual char layer, reduced the peak heat release rate by more than 80% and the smoke generation by 50% obtained from CCT. What is more, the scanning electron microscopy (SEM) showed that this flame-retardant system could promote the formation of a char layer with network structure which helps produce composites with superior flame retardant. A synergistic effect on enhancing the limit oxygen index (LOI) and restricting the dropping of the composites is also obtained. This study has a potential contribution to the development of carbon-based flame-retardant composites.