Carbon is the most important alloying element in the design of the third generation high strength steel. Characterizing the diffusion behaviors of carbon in the steel is the key to design and control the microstructure to obtain excellent mechanical properties. The research progress of carbon atom occupancy and diffusion characterization technology in medium and low-carbon structural steel was reviewed, and the relationship between carbon atom occupancy state and diffusion behavior and microstructure properties in the tempering process of medium and low-carbon structural steel was analyzed. The related characterization techniques were sorted out and compared to provide reference for the study and grasp of the relationship between the diffusion movement of carbon atoms and the transformation of microstructure.