In recent years, assembling 2D layered materials into three-dimensional (3D) architectures has become the research focus in catalysis fields. In this study, 3D porous g-C3N4@carbon (CN@C) microflowers composed of carbon-coated g-C3N4 nanosheets were prepared by a facile bio-inspired method. At first, cyanuric acid-melamine (CAM) supermolecules were assembled into 3D microflowers with the mediation of sucrose molecules through a hydrothermal process, and then isomorphically converted into 3D CN@C microflowers upon calcination. By varying the saccharide species, it was found that the formyl groups of saccharide molecules and their self-polycondensation behavior could govern the CAM assembly and subsequent formation of microflower structure. The resultant CN@C microflowers exhibit high oxygen evolution reaction (OER) activity with a potential of 1.68 V at a current density of 10 mA cm(-2) due to efficient mass and charge transfer, which is close to the reported value for the state-of-the-art metal catalyst IrO2/C (1.60 V, 0.1 mol L-1 KOH) and much superior to that of pure g-C3N4.