The nanocomposites of gold nanoparticles/reduced graphene oxide (AuNPs/rGO) have attracted a lot of attention due to their excellent synergic properties. However, a green and low-cost synthesis method without suffering from solvent toxicity, labor-consuming, or high-temperature processes is still highly desirable. In this work, we demonstrated the preparation of AuNPs/rGO nanocomposite films by a one-step dual reduction of graphene oxide (GO) and chloroauric acid (HAuCl4) using argon (Ar) plasma. The oxygen functional groups on GO anchored the Au ions and the electrons generated in Ar plasma reduce these ions into metallic Au. By controlling the time of plasma treatment and concentration of HAuCl4 precursor, the size and density of AuNPs on GO can be widely tuned. The underlying mechanism for the size- and density-tunability was also discussed. The AuNPs/rGO nanocomposite films were used as Raman substrates and achieved strongly enhanced signals with excellent reproducibility and stability. The nanocomposites were also successfully applied to the catalytical reduction of toxic 4-nitrophenol (4-NP) into non-toxic 4-aminophenol (4-AP) with a reduction rate constant as high as 0.422 min(-1). This work paves the way towards the green and low-cost industrial production of AuNPs/rGO nanocomposites which have great potentials in a variety of application fields.