Plasmonic enhancement of photocatalytic hydrogen evolution has been achieved under visible light illumination by integrating strongly plasmonic metal particles such as gold (Au) with semiconducting materials. To understand the effect of plasmon resonance on the photocatalytic hydrogen evolution reaction (HER), in this work, we study the hydrogen evolution reaction (HER) over Au nanocages with systematically tunable localized surface plasmon resonance (LSPR) properties dispersed on an Al2O3 support and covered with multiphasic 2D MoS2 nanosheets. It is interesting to observe that by tuning the LSPR wavelength of Au nanocages, an optimal enhancement in the photocatalytic HER can be obtained over the multiphasic 2D MoS2 nanosheets. In particular, when the LSPR wavelength of Au nanocages is close to the absorption edge of MoS2 nanosheets, a 40-fold increase is observed in the HER rate regarding bare MoS2. Time-resolved transient absorption spectroscopy was conducted to explicitly identify the mechanisms behind the Au/MoS2 system. The results suggest that near field enhancement (NFE) is the dominant LSPR process in this system and a detailed explanation of the working mechanism in this system is proposed. Governed by the NFE process, the energy of the surface plasma is transferred from Au nanocages to MoS2 nanosheets to promote electron–hole excitation in MoS2, and the efficiency reaches the maximum when the LSPR wavelength of Au nanocages matches the MoS2 light absorption edge, resulting in a significantly enhanced photocatalytic hydrogen yield compared to the bare MoS2 nanosheets and Au/MoS2 systems where the LSPR wavelengths of Au nanocages and MoS2 nanosheet absorption edge do not match. The learning from this work provides insights into the design of highly efficient photocatalysts based on plasmonic materials.