Information space of an urban system is a virtual space based on urban geographical space, which is filled with information elements, and interconnected by information flows both longitudinally and laterally. The development of new generation information technology is upgrading both structure and level of information space. Smart city which represents the advanced stage of urban information space has characteristics of thorough perception, interconnection, intelligent application in the process of data collection, information exchange and system application performance. Based on the information space theory, "3I" information space including instrumented, interconnected and intelligent space is proposed, and a three-dimensional model of smart city is constructed to enrich the theoretical system, so it can provide the guidance for China's smart city practice.