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The pursuit of highly efficient electrocatalysts for the alkaline hydrogen evolution reaction (HER) is of paramount importance for water splitting. However, it is still a formidable task in Mo 2 C-based materials because of the agglomeration and strong Mo-H binding of Mo 2 C units. Herein, a novel CeOCl-CeO 2 /Mo 2 C heterostructure nesting within a three-dimensional porous nitrogen-doped carbon matrix has been designed and used for catalyzing HER via simultaneous morphology and heterointerface engineering. As expected, the optimal CeOCl-CeO 2 (0.2)/Mo 2 C@3DNC exhibits impressive HER activity, with a low overpotential of 156 mV at a current density of 10 mA cm -2 coupled with a slight Tafel slope of 62.20 mV dec -1 . Introducing a Ce promoter, that is CeOCl and CeO 2 , would endow the interface with an internal electric field and electron redistribution between CeOCl-CeO 2 and Mo 2 C induced by the heterogeneous work function difference. Moreover, experimental investigation and density functional calculations confirm that the CeOCl-CeO 2 /Mo 2 C heterointerface can downshift the d-band center of the active Mo center, weakening the strength of the Mo-H coupling. This proposed concept, engineering Ce-based promoters into active entities involved in the heterostructure to modulate intermediate adsorption, offers a great opportunity for the design of superior electrocatalysts for energy conversion.