Abstract:China is experiencing a period in fast economic development and swift urbanization at present and meanwhile bearing enormous pressure from environment deterioration. Most of the cities have been suffering from such serious urban problems as environmental pollution, crowded space, low greening ratio and chaotic order. As the one of most vibrant economic-developing areas in China, the contradiction between urbanization and ecological security in Kunshan is more outstanding. Based on the summary of domestic and international ecological security and urban ecological study, this paper makes an analysis of the effects of urban land use extension on urban ecological security. Urban land use extension of Kunshan has undergone four stages, namely slow development stage (1949-1977), old town alteration stage (1977-1985), new area developing stage (1985-1999) and fast extension stage (1999 till present). From the aspects of urban waste water, waste gas and solid wastes, the paper makes an analysis of the eco-environmental change during the process of urban land use extension in Kunshan and points out that fast industrialization and urbanization process as well as fast economic development may threaten the urban ecological security in Kunshan. Meanwhile, the paper sets up an evaluation mode for urban ecological security, constructs the indexes system of urban ecological security evaluation, discusses the method of urban ecological security evaluation and evaluates the urban ecological security from 1985 to 2004 in Kunshan. At last the paper makes an alerting research on the future urban ecological security in Kunshan. In order to improve the ecological security of Kunshan and the economic sustainable development, some beneficial suggestions and countermeasures were put forward.