With the rapid development of China, urban sprawl has become a major issue in many metropolitan areas. However, the definition of urban sprawl is still controversial and its impact on urban development has not been identified clearly. The authors consider that urban sprawl refers to the uncontrolled or scattered development in suburban areas which leads to increasing traffic problems, resource depletion and open space destruction. Therefore, it is necessary to explore the urban extension process and the driving force of urban sprawl. This research aims to analyze the driving force of urban extension through the logistic regression analysis. The remote sensing and geographic information system are adopted in the research. Based on the Landsat TM/ETM images of Xi'an city in 2002 and 2007, the authors have the construction land information. Therefore, they can draw out the construction land map with the GIS technology and the sprawl information. Based on the analysis of road, river, land subsidence, ground fissure, population and GDP, the authors build up the logistic regression model to simulate the urban extension of Xi'an city. With the logistic regression model, the researchers obtain a simulation map for the urban extension of Xi'an city. The result shows that the areas of construction land are increasing that is consistent with the factual urban extension. Therefore, the special models can identify the main driving forces of urban extension in Xi'an city through validation of the logistic regression model. The authors point out that the accessibility of the roads is the most significant driving force of urban extension. Finally, the authors also note that there are many factors which can restrict the application of the urban extension model because the urban sprawl is a complex phenomenon. Therefore, the advanced urban extension model should be established with the consideration of government policies and residents' behaviors.