Abstract Graphdiyne (GDY), an emerging 2D carbon allotrope, holds immense potential for diverse applications but is severely constrained by relatively complex and time‐intensive synthesis methods. Here, a novel electron beam irradiation strategy is reported that enables the ultrafast and scalable synthesis of GDY directly from its protected monomer, hexakis[(trimethylsilyl)ethynyl]benzene (HEB‐TMS), under ambient conditions. To the best of our knowledge, this represents the first report of the direct use of electron beam irradiation in carbon materials synthesis, achieving the shortest synthesis time for GDY from HEB‐TMS reported to date. This unprecedented efficiency arises from the rapid in situ formation of copper acetylide intermediates, followed by electron‐induced homolytic cleavage to generate alkynyl radicals that undergo efficient homo‐coupling into GDY. Moreover, this flash approach enables the in situ formation of uniformly dispersed Cu 2 O nanoparticles on GDY, resulting in a composite with exceptional efficiency and stability for the electrochemical nitrate reduction to ammonia. By providing a green, scalable, and efficient synthetic route, this work not only marks a leap toward GDY production but also establishes a versatile platform for designing GDY‐based catalysts, paving the way for broader applications and industrial‐scale production.