<p>The high grinding temperature during grinding hardening causes changes in the carbon (C) potential of the hardening layer of structural steel, significantly influencing the mechanical properties of workpieces. However, existing studies on grinding hardening rarely focus on the quantitative analysis of C potential in the hardened layer, and lack a theoretical model that couples C potential field with material performance, which constitutes a key gap in optimizing the grinding hardening process. To address this, this paper establishes a C potential field model for the grinding hardening layer by integrating the grinding hardening thermal field and Fick’s laws of C diffusion in the grinding contact layer. The mechanism of decarburization and the characteristics of the C potential field are systematically analyzed: the surface layer exhibits a larger C gradient than the inner layer; due to the decarburization threshold effect, C content decreases nonlinearly with increasing grinding depth; and C content increases slightly with increasing feeding speed. Additionally, this study quantifies the influence of the C potential field on surface hardness: high grinding temperatures lead to increased C loss, reducing martensite formation and thus limiting hardness improvement. Grinding hardening experiments verify that the hardened layer undergoes phase transformation with complex grain boundaries, and the measured C content and hardness values are consistent with the model calculations. This work fills the theoretical gap in C potential prediction for grinding hardening layers and provides a quantitative basis for optimizing grinding parameters to improve the quality of hardened structural steel components.</p>