Precise atomic-level control of composition and geometric structure at the interface between two catalyst components can effectively tune the catalytic properties. Herein, we found a "chimney effect" formed on the interface between the metal oxide and Nickel metal for the hydrogen evolution reaction, using density functional theory calculations and experimental methods. This special chemical environment around the interface leads the neighboring sites to be immune to the H2O* and OH* adsorption and to only selectively adsorb H* properly. Meanwhile, it is also beneficial for the smooth adsorption of the reactant (H*) on the interface and the easy desorption of the product (H2) from the catalyst surface (AGH. close to zero). This phenomenon appears similar to a chimney of hydrogen evolution around the metal oxide/metal interface. Such "chimney effect" is a result of the interfacial charge transfer between the metal and metal oxide, and should be the nature of the interface induced synergistic effect in metal oxide/metal composite catalysts for HER. Experiments further confirm that the catalytic activity of metal oxide/metal composites for HER can be enhanced by increasing the amount of the chimney- the interfacial metal atoms.