Dispersion relations of surface plasmon polaritons(SPPS)in sandwiched optical systems are studied.The system is actually a kind of SPP waveguides,with two kinds of single negative material(SNG)as core and dadding layers.respectively.Since both TM and TE polarized SPPs can be excited in the structure,the dispersion of SPPs becomes more abundant and leads to colorful nonlinear optical properties.The authors demonstrate the effective phase-matched second and third-harmonic generation(SHG,THG)assisted by the coupled SPPs.A cascaded second-order nonlinear process can also be achieved in the structure when the thickness of the core layer is properly selected,leading to the simultaneous SHG and THG.Further investigations show that much easier phase-matching can be fulfilled in the SNG waveguide array.Our results would be of potential use for surface-enhanced frequency conversion device such as light emitters or lasers.