Metallic nanoparticles combined with 3D photonic crystals (PC) are anticipated to achieve high light-use efficiency, which stimulates their promising applications as optical antenna. Here, we propose a hierarchical optical antenna from an inverse opal photonic crystal modified with gold nanoparticles (GNPs) and exploit it for highly-sensitive bio-sensing. Metallic nanostructures can induce efficient light-harvesting and boost the light-matter interaction through localized surface plasmon resonance (LSPR). Photonic crystals are also efficient optical antennas with the ability to manipulate incident light. The 3D photonic crystal can localize the excitation energy and induce intense LSPR of GNPs for fluorescent enhancement when the PC band edge agrees with the LSPR absorption wavelength of the GNPs and the excitation wavelength of the light source. The feasibility of our approach is demonstrated by label-free DNA detection of SARS and HIV, and the GNP-modified inverse opal gives rise to one magnitude enhancement of the signal intensity and the detection sensitivity. This hierarchical antenna is an interesting prospect for highly effective light harvesting through the optimized combination of the superior properties of photonic crystals and plasmonic materials.