Bimetallic alloy nanostructures (NS) have attracted much interest compared to their monometallic counterparts for a variety of catalytic applications. Despite much progress, rapid synthesis of them has still been a big challenge. Herein, we reported a generalized strategy of rapidly synthesizing bimetallic alloy NS in a planar on-chip reactor consisting of polymer-derived porous graphene (G) via a transient laser induced process. In the process, G acts as a reactor that holds metal alloy precursors. When laser scans, G absorbs the light power and then converts it to transient localized heat to direct the nucleation and crystallization of metal alloy NS. Here, Fe-Ni alloy is chosen as a model material to demonstrate this process. In the final hybrid material of Fe-Ni/G, G serves as a conductive support, which benefits the electron transfer between the Fe-Ni alloy NS and the electrodes. The optimized Fe-Ni/G shows a superior oxygen evolution reaction (OER) activity with an overpotential of 281±4 mV at a current density of 10 mA/cm2 and a low Tafel slope of 51 mV/dec. This work would offer a universal methodology for synthesizing various metal alloy NS with controlled structures and properties, laying foundations for their widespread applications in energy storage and conversion.