The widespread Middle Jurassic to Early Cretaceous (J2-K1) intraplate tectonism of East Asia has recently been proposed to reflect the flat subduction of the Izanagi plate, whose mechanism, however, remains poorly constrained. Here, we compare these geological records with those within the western United States (U.S.) during the Laramide flat subduction. Two prominent differences are identified during slab advance: 1) western U.S. witnessed continental-scale subsidence and marine inundation, while East Asia experienced mostly basin inversion implying uplift; 2) the landward migrating magmatism in western U.S. quickly evolved into a regional magmatic lull that lasted for >40 Myr, but the East Asian magmatism was largely continuous in space and time. Assisted with numerical models, we show that a flat slab could indeed reproduce the observed East Asian crustal deformation and topographic history during the advance and retreat of the slab. We further find that 1) this slab should be more buoyant than the Laramide slab so that the former oceanic slab could break off which lead to surface uplift instead of subsidence, and 2) the crust of the flat slab melts more easily than an oceanic one. The observation that the J2-K1 tectonism was largely contemporary with the assembly (ca. 180–137 Ma) of exotic terranes along the east coast implies that the flat sab underplating North China and Northeast China likely represented subduction of the buoyant continental lithosphere associated with these terranes. The terrane slab could have carried and flattened portions of the nearby oceanic slab with it, forming a giant composite flat slab that defined the J2-K1 intraplate tectonism within East Asia.