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The intercrystalline interfaces have been proven vital in heterostructure catalysts. However, it is still challenging to generate specified heterointerfaces and to make clear the mechanism of a reaction on the interface. Herein, this work proposes a strategy of Fe‐catalyzed cascade formation of heterointerfaces for comprehending the hydrogen evolution reaction (HER). In the pure solid‐phase reaction system, Fe catalyzes the in situ conversion of MoO2 to MoC and then Mo2C, and the consecutive formation leaves lavish intercrystalline interfaces of MoO2‐MoC (in Fe‐MoO2/MoC@NC) or MoC‐Mo2C (in Fe‐MoC/β‐Mo2C@NC), which contribute to HER activity. The improved HER activity on the interface leads to further checking of the mechanism with density functional theory calculation. The computation results reveal that the electroreduction (Volmer step) produced H* prefers to be adsorbed on Mo2C; then two pathways are proposed for the HER on the interface of MoC‐Mo2C, including the single‐molecular adsorption pathway (Rideal mechanism) and the bimolecular adsorption pathway (Langmuir–Hinshelwood mechanism). The calculation results further show that the former is favorable, and the reaction on the MoC‐Mo2C heterointerface significantly lowers the energy barriers of the rate‐determining steps.

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