Iron sulfide nanomaterials represented by FeS and FeS nanozymes have attracted increasing attention due to their biocompatibility and peroxidase-like (POD-like) catalytic activity in disease diagnosis and treatments. However, the mechanism responsible for their POD-like activities remains unclear. Herein, taking the oxidation of 3,3,5,5-tetramethylbenzidine (TMB) by HO on FeS(100) and FeS(001) surfaces, the catalytic mechanism was investigated in detail using density functional theory (DFT) calculations and experimental characterizations. Our experimental results showed that the catalytic activity of FeS nanozymes was significantly higher than that of FeS nanozymes. Our DFT calculations indicated that the surface iron ions of iron sulfide nanozymes could effectively catalyze the production of HO• radicals via the interactions between Fe 3d electrons and the frontier orbitals of HO in the range of -10 to 5 eV. However, FeS nanozymes exhibited higher POD-like activity due to the surface Fe(II) binding to HO, forming inner-orbital complexes, which results in a larger binding energy and a smaller energy barrier for the base-like decomposition of HO. In contrast, the surface iron ions of FeS nanozymes bind to HO, forming outer-orbital complexes, which results in a smaller binding energy and a larger energy barrier for the base-like decomposition of HO. The charge transfer analysis showed that FeS nanozymes transferred 0.12 e and FeS nanozymes transferred 0.05 e from their surface iron ions to HO, respectively. The simulations were consistent with the experimental observations that the FeS nanozymes had a greater affinity for HO compared to that of FeS nanozymes. This work provides a theoretical foundation for the rational design and accurate preparation of iron sulfide functional nanozymes.