Self-assembled monolayer film of gold nanoparticles (55 nm) was formed at air-water interface by the driving force of wettability-shift of gold nanoparticles from hydrophilic property to hydrophobic property when encapsulated with 1-dodecanethiol. SEM image shows that the structure of the surface is nearly monolayer with closed arrays of uniform particle size when the film is transferred onto Si wafer. The substrate can be used for SERS substrate to realize semiquantitative analysis of melamine and the detection limit can reach 1×10-9 g·L-1. This SERS substrate is of wide application, not only for melamine but also for nonpolar molecule such as polycyclic aromatic hydrocarbons.