Abstract: With the growing social needs and the rapid development of information technology, digital city made remarkable progress, which provided well service for government, enterprises and the public. But there exist a lot of problems. The emergence and the application of grid, the internet of things and cloud computing brought opportunity for solving these problems. Smart city emerges as the times require. The evolution from digital city  to smart city is now  the social topics. Grid, the internet of things and cloud computing are key technologies which realize the evolution from digital city to smart city. Smart city will promote the development of smart service and smart industry, and enhance city’s core competitiveness in the 21st century.