Calculating and assessing available land (reserved construction land potential) is a key foundation of Major Function Oriented Regionalization. Under the background of land resources scarcity and the policy of cultivated land conservation, precisely evaluating land resources, especially reserved construction land of different regions seems to be one of important issues on how to put forward Major Function Oriented Regionalization. The article puts emphasis on definition, function, factor parameters, and arithmetic and technique course of available land for Major Function Oriented Regionalization. With the help of GIS spatial analysis techniques, using Digital Elevation Map (DEM), TM remote sensing land use map of 2000, administrative map at county level and land use investigation data of 2005, the article calculated and assessed quantity and spatial differentiation of available land of China in 2005. The results show that: available land area for construction in China amounts to 28.54×104 km2, which accounts for 3.01% of the total land area. Available land area per capita is 0.34 mu/person, and the main characteristics of spatial differentiation show that: the high value areas are concentrated in Northeast China, northern part of North China, Northwest China and the middle reaches of the Yellow River, while the low value areas are aggregated on the Qinghai-Tibet Plateau and densely populated areas of Central and Eastern China. Available land is a comprehensive index for assessing land capacity of population agglomeration, industrialization and urbanization in different regions. However, as a lot of factors are related to the index, one single index (available land area or available land area per capita) cannot precisely reflect all the contents of the index of available land. In the process of practical application, a few issues should be concerned. Firstly, available land index items consist of three factors: quantity, quality and scale of massing of reserved construction land. Available land area and available land area per capita are two key indices based on the single factor method. Secondly, supplementary indices should be set up, such as abundance of available land per capita, abundance of available land, and etc. Finally, according to the definition of the index, available land should be derived from residual available land, potential available land and adjusting available land. The location of the study area, work targets, scale of the study area and data accessibility should also be focused in the practical process of calculating and assessing available land.