The combination of GIS and clouding computing provides an extensive development prospect for GIS information storage, management, processing and application. The definition of cloud computing and cloud GIS are presented, four application mode and a multi-level architecture model of cloud GIS are proposed. The key techniques, such as virtual technology, distributed data storage technology (Google File System), virtual data management technology (BigTable), parallel spatial analysis technology, data and functional interoperability technology as well as deployment model are mainly studied. Finally, an actual application framework of cloud GIS is given.