Achieving the Sabatier optimal of a chemical reaction has been the central topic in heterogenous catalysis for a century. However, this ultimate goal is greatly hindered by the extrinsic modifications in previous catalyst design strategies. Accord-ing to the magneto-catalytic effect (MCE), the performance of ferromagnetic catalysts can be promoted without changing its chemical structure. Herein, we use time-dependent density functional perturbation theory (TDDFPT) calculations to elucidate that a partially demagnetized ferromagnet could be a Sabatier optimal catalyst. Using ammonia synthesis as the model reac-tion, we determined the activity of Cobalt at each demagnetized state by including the magnetically thermal excitations via magnon analysis, making the 55% demagnetized Co to the genuine Sabatier optimal. As an essential but under-excavated phenomenon in heterogeneous catalysis, the MCE will open a new avenue to design high-performance catalysts.