Regulating the surface plasmon resonance (SPR) of metallic nanostructures is of great interests for optical and catalytic applications, however, it is still a great challenge for tuning SPR features of small metallic nanoparticles (< 10 nm). In this work, we design a unique dielectric support—urchin-like mesoporous silica nanoparticles (U-SiO₂) with ordered long spikes on its surface, which can well enhance the SPR properties of ~ 3 nm gold nanocrystals (AuNCs). The U-SiO₂ not only realizes the uniform self-assembly of AuNCs, but also prevents their aggregation due to the unique confinement effect. The finite-difference time-domain simulations show that the AuNCs on U-SiO₂ can generate plasmonic hot spots with highly enhanced electromagnetic field. Moreover, the hot electrons can be effectively and rapidly transferred through the interface junction to TiO₂. Thus, a high visible-light-driven photocatalytic activity can be observed, which is 3.8 times higher than that of smooth photocatalysts. The concept of dielectric supports engineering provides a new strategy for tuning SPR of small metallic nanocrystals towards the development of advanced plasmon-based applications.