Urban geology has been developed for nearly one hundred years, and great progress has been made in subject property research. However, in general, there are many controversies about the subject property. In addition, it is difficult for the basic theory of urban geology to meet the needs of modern urban management and hence it can not provide a systematic and complete geoscience solution for high-quality urban development. In this paper, the development and research progress of urban geology were systematically studied. The subject property of urban geology was further clarified and a concept was put forward for urban geological system. On the basis of this concept, the research object of urban geology was clearly defined, and the research content and technical framework of modern urban geology were described. The result obtained by the authors provides a research basis for the formation of a perfect theoretical system of urban geology in the future.