"Compact city" is urban space development pattern of preventing spread of city, saving utilization land and energy, improving urban efficiency with high residential density of less dependent on motor vehicle and land mixed use.Base on the definition of connotation of "compact city", we explore the city compactness space evolution rule.We proposes 3D index of comprehensive measures of compact degree and analysis scale compact degree, form compact degree, function compactness degree and space integrated compact degree.Studies show that compact degree and topographic distribution has obvious correlation.High degree of compact unit most of these are located in the plain area and low compact degree of village and township most is located in the mountain area.Space compactness present "center-peripheral" decline characteristics and center of Xicheng district and Dongcheng district are the highest degree of compact space and Chaoyang district, Haidian district, Shijingshan and Fengtai space compactness gradually decreasing.Other suburbs are the lowest degree.The space compact degrees of eastern region higher than the west, south higher than the north.Space compactness and economic development level has obvious correlation. <<