In this paper, the method of molecular dynamics simulation is used to study the Cu-Ar nanofluid. It is found that when Cu nanoparticles of different volume fractions are added to liquid Ar, the thermal conductivity of the nanofluid can increase by 32.1%; adding different sizes of Cu nanoparticles. In the case of particles, the thermal conductivity of nanofluids can increase by 15.2%. On this basis, the strengthening mechanism of the thermal conductivity of the nanofluid was analyzed from a microscopic point of view, and it was found that the microstructure of the base fluid has changed after adding nanoparticles. The degree of change of the microstructure of the base fluid is greatly affected by the size of the nanoparticles. The closer to the nanoparticle base fluid, the more significant the microstructure changes. The thickness of the adsorption layer was calculated by dividing the surface of the nanoparticle and tracking the movement trajectory of Ar atoms in these areas and counting changes in the number of argon atoms. It was found that the adsorption layer was a key factor leading to the improvement of the thermal conductivity of nanofluids.