Heteroatom-doped carbon nanomaterials have attracted significant attention as anode materials for sodium-ion batteries (SIBs). Herein, we demonstrate a conjugated polymer-mediated synthesis of sulfur and nitrogen co-doped carbon nanotubes (S/N-CT) via the carbonization of sulfur-containing polyaniline (PANI) nanotubes. It is found that the carbonization technique greatly influences the structural features and thus the Na-storage behavior of the S/N-CT materials. The carbon nanotubes developed using a two-step carbonization process (heating at 400 ℃ and then at 900 ℃) exhibit a high specific surface area, enlarged interlayer distance, small charge transfer resistance, enhanced reaction kinetics, as well as a large number of defects and active sites; further, they exhibit a high reversible capacity of 340 mAh·g^–1 at 0.1 A·g^–1 and a remarkable cycling stability with a capacity of 141 mAh·g^–1 at 5 A·g^–1 (94% retention after 3, 000 cycles). Direct carbonization of conjugated polymers with a specific morphology is an eco-friendly and low-cost technique for the synthesis of dual atom-doped carbon nanomaterials for application in energy devices. However, the carbonization process should be carefully controlled in order to better tune the structure–property relationship.