The invention provides application of a carbon nitride nanosheet to the improvement of the ageing resistance of asphalt. The preparation method of the carbon nitride nanosheet comprises the following steps: ensuring that melamine and urea or dicyandiamide is subjected to high-temperature heat polymerization for 4 to 5 h at 550 to 600 DEG C, so as to obtain layered carbon nitride powder, grinding the powder, sieving the powder through a 200 to 500-mesh sieve, adding the powder into a stabilizer, performing high-speed shearing for 2 to 4 h at 6000 to 8000 r/min, and drying the powder, so as to obtain the carbon nitride nanosheet. The carbon nitride nanosheet is stirred and mixed with matrix asphalt, so that modified asphalt is obtained. Compared with the prior art, the application has the advantages that the carbon nitride nanosheet belongs to a two-dimension nanometer, and is layered and fused with the asphalt, and an asphalt composite material is made from the carbon nitride nanosheet serving as a reinforced phase, so that the asphalt composite material presents excellent the ageing resistance.