Abstract: In recent years, with increasing urban population, the conditions of communications and accommodations become more and more crowded. In some medium- sized or huge cities in our country, the number of underground engineering projects, such as communication tunnels,subways and underground markets increases year after year. Because of the pecularities of urban engineering circumstance and the characters of underground engineering projects, a series of environmental geotechnical problems, such as instability of foundation pit, ground subsidence,karst collapse, instability of cave and water irruption become more conspicuous and frequent. They seriously affect not only the construction of underground engineering projects and safety of buildings around, but also cause huge economic loss to the whole country. The problems have attract attention of experts in the geotechnical area and become the key study subject in the area. In combination with the practical constructions in Nanjing, the authors discuss the characters and mechanism of environmental geotechnical problems of urban underground engineering. At the same time, some suggestions and methods for solving the environmental geotechnical problems are given in the paper.