With rapid development of the new generation information technology,digital city is evolving to smart city that creates a new vision for modern city.Smart city is a concrete manifestation of smart earth in city areas,and it has three features of thorough perception,depth interconnected and intelligent application.This study compares digital city and smart city from various aspects of data collection,interconnection,information processing,architecture and application,based on which the technology roadmap of instrumented,interconnected,intelligent were discussed.