Finding catalysts and mechanisms for efficient electroreduction of N2 into NH3 remains of great challenge, due to the limitation of the scaling relations and competing HER. Here we report that N2 can be dissociated and dynamically hydrogenized on a peculiar sub-monolayer MoS2-x structure. Different with conventional surface-binding paradigms of the reaction intermediates, such a dynamic surface-binding paradigm can selectively stabilize the nitrogen intermediates and enable the circumvention of the inherent scaling relations in electrocatalysis of N2 to NH3. Atomic resolution AC-STEM and DFT calculations confirm the atomic arrangement and a ���3% compressive strain in the large area of in-plane S-vacancies of sub-monolayer MoS2-x structure. Further experimental and theoretical results demonstrate that the as-designed surface tune their catalytic properties through adjusting their surface-binding to nitrogen intermediates with tunable nitrogen affinity, leading to outstanding electrocatalytic NRR performance at ultra-low overpotential (���0.2 V. vs. RHE).