Formation of plasmonic structure in closely packed assemblies of metallic nanoparticles (NPs) is essential for various applications in sensing, renewable energy, authentication, catalysis, and metamaterials. Herein, a surface-enhanced Raman scattering (SERS) substrate is fabricated for trace detection with ultrahigh sensitivity and stability. The SERS substrate is constructed from a simple yet robust strategy through in situ growth patterned assemblies of Au NPs based on a polymer brush templated synthesis strategy. Benefiting from the dense and uniform distribution of Au NPs, the resulting Au plasmonic nanostructure demonstrates a very strong SERS effect, while the outer polymer brush could restrict the excessive growth of Au NPs and the patterned design could achieve uniform distribution of Au NPs. As results, an ultra-low limit of detection (LOD) of 10⁻¹⁵ M, which has never been successfully detected in other work, is determined for 4-acetamidothiophenol (4-AMTP) molecules and the Raman signals in the random region show good signal homogeneity with a low relative standard deviation (RSD) of 7.2%, indicating great sensitivity and reliability as a SERS substrate. The LOD values of such Au plasmonic nanostructures for methylene blue, thiram, and R6G molecules can also reach as low as 10⁻¹⁰ M, further indicating that the substrate has a wide range of applicability for SERS detection. With the help of finite difference time domain simulations (FDTD) calculation, the electric field distribution of the Au plasmonic nanostructures is simulated, which quantitatively matches the experimental observations. Moreover, the Au plasmonic nanostructures show good shelf stability for at least 10 months of storage in an ambient environment, indicating potentials for practical applications.