In recent years, two-dimensional (2D) group VA elemental materials have attracted considerable interest from physics/chemistry and materials science communities, with particular attention paid to honeycomb blue phosphorene. To date, phosphorene is limited to its α-phase and small sizes because it can only be produced by exfoliating black phosphorus crystals. Here, we report the direct synthesis of high-quality phosphorene on a nonmetallic copper oxide substrate by molecular beam epitaxy. By combining scanning tunneling microscopy/spectroscopy, X-ray photoelectron spectroscopy, and first-principles calculations, we demonstrate the growth intermediates and electronic structures of phosphorene on CuO/Cu(111). Surprisingly, the grown phosphorene has a flat honeycomb lattice, similar to graphene, which exhibits a metallic nature. We reveal that the growth mechanism and morphology of phosphorene are strongly correlated with the surface structures of prepared copper oxide, and the resulting phosphorene can be stabilized after high-temperature annealing above 600 K even in oxygen gas. The high stability is closely related to the irregular Moiré pattern and structural corrugations of phosphorene on CuO/Cu(111) that efficiently relieve the surface strain. These results shed light on future fabrication of large-scale, versatile 2D structures for interconnect and device integration.