Abstract: Data has been playing an increasingly important role in building intelligent cities in the last decade. Recently, the advances in artificial technology has boosted a rising trend of using knowledge mined from data rather than raw data to tackle urban challenges. Just like designing data management frameworks for intelligent cities in the last decade, we need to define the knowledge management framework for the coming new era, in order for that knowledge generated by different applications can be constantly accumulated and consistently shared between each other. Otherwise, new knowledge islands will be created while we are spending a lot of efforts to break down data islands. To address this issue, we propose the knowledge system, consisting of the definition of the content of knowledge in intelligent cities and the framework of knowledge representation, generation and application. The knowledge system can help professionals to acquire domain knowledge quickly, mine knowledge from data efficiently, apply knowledge to solve problems effectively, and share knowledge between each other consistently, thus leading to smarter and greener cities.