Two-dimensional (2D) mesoporous nanomaterials are required in catalysis, separation, adsorption, and energy storage fields due to their outstanding mass transfer performance. However, their fabrication via the 'bottom up' strategy has been rarely reported and is limited by the difficulties in obtaining a versatile and accessible structure-directing agent. Here, ultrathin mesoporous silica nanosheets (MSN) were successfully synthesized by employing acidified g-C3N4 as a structural directing agent owing to its natural layered structure, stoichiometric solubility, and amphiphilicity. The thickness of MSN is readily adjustable by tuning the dosage of acidified g-C3N4 during the fabrication process, and when the mass ratio of silica/acidified-g-CzN4 is 10, the thickness of the MSN is 6-9 nm. TEM, SAXRD, and BET analysis demonstrated the mesoporous characteristics of MSN with a long-range ordered hexagonal arrangement symmetry, a uniform pore size distribution around 2.9 nm, and high BET surface areas of 1000-1150 m2 g-1. The superior mass-transfer performance of MSN in catalysis applications, which was derived from its special structure, was confirmed by the outstanding methane combustion activity of MSN supported Co3O4 catalysts. This work provides a controllable and scalable 'bottom up' fabrication method for 2D porous material, and also opens up an alternative application for g-C3N4.