As photocatalysts, holey carbon nitride nanosheets (HCNS) have attracted considerable attention. Although many advances have been made in the preparation of HCNS, challenges remain in the low-cost preparation of HCNS. In this work, a one-step strategy for preparing HCNS was explored. First, melamine was placed in a small ceramic boat, which was in a large ceramic boat. Next, the gap between two ceramic boats was filled with copper oxide. Finally, the two boats were wrapped in tin foil and heat-treated. During the thermal polymerization of melamine, released ammonia is decomposed by copper oxide, and the equilibrium shifts to the right, further accelerating the release of ammonia and achieving the preparation of HCNS. Meanwhile, partial copper oxide is reduced to copper by ammonia. After a simple heat treatment, the copper in the mixture can be converted back to copper oxide and realize recycling. The obtained HCNS displays high looseness and enhanced photogenerated charge carrier separation efficiency. Our results indicate that the filled copper oxide can effectively decompose ammonia, affect the thermal condensation atmosphere, and significantly improve the photocatalytic activity of carbon nitride. This work provides a new idea for further fundamental and applied research of HCNS.