The high-precision characterization of shallow subsurface three-dimensional structures is a frontier topic for ensuring the safe development of urban underground space. The use of environmentally friendly, non-invasive geophysical methods to finely identify underground structures is a pressing issue that needs to be addressed. This paper takes the underground fine structure detection in Xiong'an New Area as an example, using the microtremor H/V spectral ratio method to probe the internal structure of thick sedimentary layers. The morphological characteristics of the H/V spectral ratio curve directly reveal the form of the thick sedimentary layer in Xiong'an New Area. Its bimodal shape represents the lithological interfaces of the underground structure in the area: the high-frequency peak corresponds to the interface between silty soil and clay; the low-frequency peak corresponds to the interface between the Quaternary and Neogene layers, with the associated lithology inferred to be silty clay and sandstone or gravel sandstone. The S-wave velocity structure obtained by inversion of the H/V spectral ratio curve using a particle swarm algorithm shows that the inversion results align with the actual stratigraphic structure, with a certain resolution for weak inter-layers. Experimental results demonstrate that the microtremor H/V spectral ratio method provides a scalable technical paradigm for urban geological transparency.