Urbanization is developing rapidly in the world, which seriously changes the habitat of organisms and has clearly a negative effect on biodiversity. Preservation of biodlversity is crucial in urban planning and management, which is also an important symbol for the level of greening. Problems such as scarcity of urban green space and plant species have become obstacles to the establishment of ecological friendly cities. However, coexistence of nature and modernization, as well the coordination of economic development and biodiversity, are goals that people are seeking. We have taken the built-up areas of Beijing as a study case and discussed the impacts of urbanization on plant diversity, with the support of fieldwork and SPOT remote sensing data. The results are as follows: 1) in the process of urbanization, exotic plants have been widely introduced, which has affected species composition and the proportion of native plants; it is clear that artificial green spaces always will have a lower level of plant diversity than natural green spaces; 2) functional differences of green space types partially decide their species abundance, so that plant diversity in greenbelts and streets is generally lower than in parks; 3) the spatial variety of plant diversity contributes much to the imbalance of district development and the planning of different functional zones; this variation is embodied in different ring-belts and directions; 4) habitat fragmentation also affects plant diversity to a great extent; there is a significant positive correlation between high fragmentation and low plant diversity. According our results, some suggestions are proposed, which would be suitable for the preservation of plant diversity and ecological improvement during urbanization.