This paper is focused on an introduction to the mechanism of nonlinearity enhancement by field confinement and magnification effects of surface plasmon polariton (SPP). By coupling the incident electromagnetic field with the coherent motion of free-electron plasma in the metal, SPP is excited near the metal surface, providing field confinement in nanoscale, which results in the enhancement of electrical field and nonlinearity magnitude. The light intensity required for nonlinear process is dramatically reduced and the occurrence of weak light nonlinear process in nanoscale is possible. Starting from the situation of metallic nanoparticle system, the basic principle of enhancement of electric field and nonlinearity by surface plasmon resonance is introduced. A theory for the evaluation of third-order optical susceptibility of nonlinear nano-composites is presented, which is further used to analyze the nonlinear property of an asymmetric split ring metamaterial. Finally, the surface plasmon resonance is used to enhance the magnitude of nonlinear optical activity effect. The nonlinearity enhancement by surface plasmon is proved to pave a way for the development of weak light nonlinearity in nanoscale.