In order to solve the common problems of human-machine-things fusion for the "complex giant system" running of city, construct a universal platform for smart city service, combining with the basic research of smart system theories, analyzed the practical application of smart system in smart city, such as IBM, Ali,HUAWEI e?Hualu. It cleared the basic connotation and characteristics of smart system, the general composition and system architecture of smart system is designed. From the four system units design technologies of sense, transmission, wisdom and application, and system overall design technology, the composition and application direction of advanced technologies in smart system for smart city application are comprehensively expounded, which provides technical guidance and system construction reference for the designing of smart system.