Urban underground space (UUS) is rapidly evolving from a "supplementary space" into a "fourth national territory" that supports the high-quality development of future cities. However, UUS faces key challenges, including lagging spatial planning, insufficient human-centered design, low environmental quality, and limited technological integration. In this context, architecture, as a discipline centered on human habitation and spatial design, plays a pivotal role in enhancing the quality, adaptability, and sustainability of underground environments. This study defines the research scope of UUS from an architectural perspective, identifies its strategic importance and development drivers, and reviews both international research progress and the developmental trajectory of underground architecture in China. It proposes a three-dimensional research framework centered on "multi-layer planning, spatial creation, and technology integration and evaluation", and highlights critical research topics that need breakthroughs, covering spatial strategies for intensive and coordinated development, human-centered design approaches, digital support tools, and scientific evaluation systems. Furthermore, the study explores methodological innovations and application pathways, such as architect-friendly simulation tools for early design stages, multidimensional perception-based built environment evaluation methods, optimization of UUS safety resilience and peacetime-wartime transition mechanisms, and intelligent operation and maintenance technologies for UUS. By establishing architectural theories and design systems for complex systems, advancing the use of deep UUS, exploring human-centered and resilient design, improving multidimensional evaluation systems and standards, and promoting performance-driving architectural design integrating simulation, optimization, and evaluation, this study can provide a comprehensive foundation for the high-quality and sustainable development of future UUS.