Semiconductor nanostructures (e.g. nanowires and nanobelts) hold great promise as sub-wavelength coherent light sources, nonlinear optical frequency converters and all-optical signal processors for optoelectronic applications. However, at such small scales, optical second harmonic generation (SHG) is generally inefficient. Herein, we report on a straight-forward strategy using a thin Au layer to enhance the SHG from a single CdS nanobelt by 3 orders of magnitude. Through detailed experimental and theoretical analysis, we validate that the augmented SHG originates from the mutual intensification of the local fields induced by the plasmonic nanocavity and by the reflections within the CdS Fabry-Pérot resonant cavity in this hybrid semiconductor-metal system. Polarization-dependent SHG measurements can be employed to determine and distinguish the contributions of SH signals from the CdS nanobelt and gold film, respectively. When the thickness of gold film becomes comparable to the skin depth, SHG from the gold film can be clearly observed. Our work demonstrates a facile approach for tuning the nonlinear optical properties of mesoscopic, nanostructured and layered semiconductor materials.