The invention discloses a centimeter-level and adjustable-interparticle-distance polymer grafting gold nanoparticle composite single-layer film and a preparation method thereof. The composite single-layer film is formed by orderly arranging polymer grafting gold nanoparticles in a hexagonal packing mode. Oil-soluble polymers are used for performing grafting modification on the surfaces of the gold nanoparticles, and are dispersed in an organic solvent; the oil-soluble polymer grafting gold nanoparticles are spontaneously spread into a film on the water surface by an oil/water interface volatilization method; finally, the film is transferred to the surface of a carrier base plate, so that the required composite single-layer film is obtained. The preparation method is simple in operation, has high repeatability and is applicable to nanoparticles in different sizes and different oil-soluble polymers; the large-scale preparation is convenient. The obtained polymer grafting gold nanoparticle composite single-layer film has the advantages that the nanoparticle arrangement is order; the interparticle distance is controllable; the maximum area can reach 10cm<2>; the polymer grafting gold nanoparticle composite single-layer film is hopeful to be applied in photoelectric devices, microelectronic devices and micro-nano structure sensors.