This paper studies urban spatial expansion of Lanzhou City, Gansu Province of China based on Geographical Information Systems (GIS). The study explores the temporal and spatial characteristics of urban spatial expansion from 1990 to 2004. According to Lanzhou City's land-use maps in 1990, 1995 and 2000, and the satellite images in 2004, the authors gain the information of Lanzhou City spatial expansion Since 1990. By Annual Growth Index(AGI), the authors analyzed the spatial differentiation of urban spatial expansion in Lanzhou City in three study periods, including 1990-1995, 1995-2000 and 2000-2004. It is found, spatially, during 1990-1995, the mean annual growth index was 1.20, with the maximum of 18.67, the expansion areas with a high speed of more than 7.56 were mainly distributed in Anning District; during 1995-2000, the mean annual growth index was 0.35, with the maximum of 3.84, the expansion areas with a high speed of more than 2.53 were mainly distributed in Yantan District, south of Qilihe District and northwest of Anning District; during 2000-2004, the mean annual growth index was 2.87, with the maximum of 18.90, the expansion areas with a high speed of more than 10.71 were mainly distributed in Yantan District and Anning District.
Temporally, the research shows: 1990-1995 is the fast growth stage, with the average annual expansion rate of 349.76 ha/year; 1995-2000 is the crawling growth stage, with the average annual rate of expansion 102.26 ha/year; 2000-2004 is the high-speed growth stage, with the average annual expansion rate of 836.67 ha/year. Major urban expansion is in Chengguan District and Anning District, and there are sprawl expansion process and in-filled expansion process. According to elastic coefficient of urban expansion, the authors think Lanzhou City spatial expansion is too rapid. Finally, the authors talk about the mechanism of Lanzhou City spatial expansion, including the natural environment, economic factors, social factors, policy factors, system factors and other factors. Focusing on analysising economic and social factors by related analysis method, the paper concludes that The main driving factors of urban sprawl are population, the length of roads in urban area, capital investment in fixed assets, the total output value of industrial enterprises and the average salary. Although there are many driving factors, the authors believe the mean salary is the most leading factor. This shows that the rapid economic growth is the decisive factor of urban expansion.