Under light irradiation, gold nanoparticles (AuNPs) reveal the surface plasmon feature, i.e., the occurrence of the collective excitation of the free electrons of NPs. Plasmon relaxation, as well as excitation, induced by light absorption, could be used to increase the local temperature via conversion of light to heat. This photothermal effect can be enhanced by control of the morphology and structure of NPs in the near-infrared (NIR) region. Recently, the use of an NP-composited polymer as a heating patch with a good photothermal performance was suggested for biomedical applications. Herein, AuNPs embedded on polydimethylsiloxane (PDMS) films (Au-PDMS) were successfully prepared with an in-situ synthesis method without a reducing agent. Their photothermal performance was measured with an IR camera under 808 nm NIR irradiation, and a mechanical stretching test for the Au-PDMS films was conducted to investigate the effect of the AuNPs’ density on the photothermal performance. The surface temperature of the films, which reached 120 °C within 1min, is also adjustable with mechanical stretching (strain change). This is due to the decrease of the AuNPs density with widening interparticle distance between them.