Currently, the direct synthesis of inch-scale single-crystal graphene on insulating substrates is limited by the lack of metal catalysis, suitable crystallization conditions, and self-limiting growth mechanisms. In this study, we investigated the direct growth of adlayer-free ultra-flat wafer-scale single-crystal monolayer graphene on insulating substrates by the multi-cycle plasma-etching-assisted chemical vapor deposition (MPE-CVD) method. Firstly, an angstrom-scale growth nanochamber was created by fabricating single-crystal Cu(111) foils on Al2O3(0001) substrates. Graphene was then directly synthesized at the interface between Cu(111) and Al2O3(0001) by MPE-CVD. After growth, the Cu(111) foil was detached using a liquid-nitrogen-assisted separation method, and the ultra-high-quality single-crystal graphene film was experimentally achieved on Al2O3(0001). This work breaks the bottleneck in the direct synthesis of single-crystal monolayer graphene on insulating substrates and paves the way for next-generation carbon-based atomic electronics and semiconductor nanodevices.