The possible phase transformation of catalysts under reaction conditions brings lots of difficulties in establishing the active phase. Herein, we report a hidden active phase over iron catalyst uniquely for the dehydrogenation of ammonia (NH₃). The highest dehydrogenation rate corresponds to an evanescent Fe/Fe₄N mixing phase while the nitrogen (N) kept on accumulating and gradually deactivated the catalyst. Density functional theory (DFT) calculations demonstrated that deposition of an N on Fe(100) surface modifies the electronic structure of its surrounding iron atoms, causing a significant reduction of the initial dehydrogenation barrier of NH₃. To recover the hidden active phase, ambient-pressure double dielectric barrier discharge (DDBD) plasma was applied to the reaction system in situ to remove the excessive surface N, which yields a pronounced improvement of the catalytic performance. The work demonstrates that hidden active phase in thermal catalysis can be unfolded when the rate-determining step is subdued by applied plasma.